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Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

Appendix A
1995 Survey Methodology

Sample Design

The sampling frame for the Survey of Doctorate Recipients (SDR) (including the Survey of Humanities Doctorates) is compiled from the Doctorate Records File (DRF), an ongoing census of all research doctorates earned in the United States since 1920. For the 1995 survey the sampling frame comprised individuals who

  • had earned a doctoral degree from a U.S. college or university in a humanities field;
  • were U.S. citizens or, if non-U.S. citizens, indicated they had plans to remain in the United States after degree award; and
  • were under 76 years of age.

To develop the frame, graduates who had earned their degrees since the 1993 survey and met the conditions listed above were added to the frame; those who were carried over from 1993 but had attained the age of 76 (or died) were deleted. A sample of the incoming graduates was drawn and added to the panel sample that is conveyed from year to year. The total sample size was 8,829.

The basic sample design for the 1995 SDR was a stratified random sample with the goal of proportional sampling across strata. The variables used for stratification were field of degree (11 groups), gender (two groups), and year of degree (two groups, distinguishing recent graduates from all others). This resulted in 44 sampling cells.

In determining sampling rates the goal was to achieve as much homogeneity as possible while allowing for oversampling of certain small populations (e.g., minority women). In practice, however, the goal of proportional sampling was not consistently achieved. A number of sample size adjustments over the years, in combination with changes to the stratification, led to highly variable sampling rates, sometimes within the same sampling cell. The overall sampling rate was about 7.7 percent, applied to a population of 115,043. Across strata, however, the rates ranged from 5.3 to 26.5 percent. The range in sampling rates serves to increase the variance of the survey estimates.

Data Collection

Data collection was conducted through a self-administered mail survey. This consisted of two mailings of the survey questionnaire with a reminder postcard between the mailings. The first mailing was in May 1995 and the second (using Priority Mail) in July 1995. To encourage participation, all survey materials were personalized with the respondent's name and address. The mail survey achieved a response rate of about 65 percent. Because of budget constraints,

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

the 1995 survey. As a result, the response rate for the 1995 survey was lower than the rates for the two previous surveys.

Data Preparation

As completed mail questionnaires were received, they were logged into a receipt control system that kept track of the status of all cases. Coding staff then carded out a variety of checks and prepared the questionnaires for data entry. Specifically, they resolved incomplete or contradictory answers, reviewed "other, specify" responses for possible backcoding to a listed response, and assigned numeric codes to open-ended questions (e.g., employer name). A coding supervisor validated the coders' work.

Once cases were coded, they were sent to data entry. The data entry program ensured that only values within allowable ranges were entered and that built-in consistency checks were not violated. For example, a case in which a respondent reported unemployment but later gave a salary was flagged for review.

Finally, to correct for item nonresponse, data not reported by the respondent were imputed. Two imputation methods were used: "cold decking," which used historical data provided by the sample member in past surveys to fill in the missing response, and "hot decking," which used a donor with similar characteristics to provide a proxy response for the missing value.

Weighting and Estimation

The general purpose of weighting survey data is to compensate for unequal probabilities of selection to the sample and to adjust for the effects of nonresponse (see the next section for a discussion of nonresponse). Weights are often calculated in two stages. In the first stage, unadjusted weights are calculated as the inverse of the probability of selection, taking into account all stages of the sampling selection process. In the second stage, these weights are adjusted to compensate for nonresponse; such nonresponse adjustments are typically carried out separately within multiple weighting cells.

The first step in constructing an unadjusted weight for the 1995 SDR sample cases was to develop a basic weight that reflected the selection probabilities for each case. This basic weight was calculated as the inverse of the sampling rate for each case. The next step was to adjust the basic weight for nonresponse. Nonresponse adjustment cells were created using poststratification. Within each nonresponse adjustment cell, a weighted nonresponse rate was calculated. The nonresponse adjustment factor was the inverse of this weighted response rate.1

Let f be the final adjustment factor for a given cell and BSCWGT denote the basic weight for the respondents. The final weight (FINWGT) for the respondents is given by

1  

The initial set of nonresponse adjustment factors was examined, and under certain conditions some of the cells were collapsed.

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

Estimates in this report were developed by summing the final weights of the respondents selected for each analysis.

Reliability of the 1995 Survey Estimates

Because the estimates shown in this report are based on a sample, they may vary from those that would have been obtained if all members of the target population had been surveyed (using the same questionnaire and data collection methods). Two types of error are possible when population estimates are derived from measures of a sample: nonsampling error and sampling error. By looking at these errors, it is possible to estimate the accuracy and precision of the survey results. Potential sources of nonsampling error in the 1995 SDR are discussed below, followed by a discussion of sampling error—how it is estimated and how it can be used in interpreting the survey results.

Nonsampling Error

Nonsampling errors in surveys can arise at many points in the survey process, and they take different forms:

  • Coverage errors can occur when some members of the target population are not identified and therefore do not have a chance to be selected for the sample.
  • Response errors can occur either when the wrong individual completes the survey or when the correct individual cannot accurately recall the events being questioned. Response errors can also arise from deliberate misreporting or poor question wording that leaves room for inconsistent interpretation by respondents.
  • Processing errors can occur at the point of data editing, coding, or key entry.
  • Nonresponse errors can occur when some or all of the survey data are not collected in a survey year.

In the 1995 survey, coverage errors are likely to be minimal because the DRF (the sampling frame for the SDR) is considered a complete census.2 Every effort was made to assure that the wrong person did not complete the form and that questions were clear and unambiguous, which keeps response errors to a minimum. Furthermore, careful cross checking and editing reduced processing errors.

However, this leaves the largest potential source of nonsampling error—nonresponse. Nonresponse bias is defined as "the bias or systematic distortion in survey estimates occurring because of the inability to obtain a usable response from some members of the sample."3

2  

Henderson, P. H., J. E. Clarke, and M. A. Reynolds, 1996, Summary Report 1995: Doctorate Recipients from United States Universities , National Academy Press, Washington, D.C.

3  

Lessler, Judith T. and William D. Kalsbeek, 1992, Nonsampling Error in Surveys, Wiley, New York, p. 118.

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

Nonresponse bias is concerned with the "representativeness" of the respondents, that is, with how respondents' characteristics compare with those of the population from which they were chosen. If the respondents do not accurately represent the population, this would result in inaccurate population estimates.

Table A-1 shows the overall weighted response rate and weighted response rates by subgroups. The overall weighted response rate4 was 65.1 percent. By field of degree, weighted response rates ranged from 60.8 percent (doctorates in philosophy) to 69.8 percent (doctorates in American history). Subgroups defined by cohort and sex had response rates ranging from 64.0 to 67.1 percent. While the direction and magnitude of bias in the estimates derived from the survey are not known, the response rates obtained suggest that nonresponse bias may exist.

Sampling Error

Sampling error is the variation that occurs by chance because a sample, rather than the entire population, is surveyed. The particular sample that was used to estimate the 1993 population of humanities doctorates in the United States was one of a large number of samples that could have been selected using the same sample design and size. Estimates based on each of these samples would have differed.

Standard errors indicate the magnitude of the sampling error that occurs by chance because a sample rather than the entire population was surveyed. Standard errors are used in conjunction with a survey estimate to construct confidence intervals—bounds set around the survey estimate in which, with some prescribed probability, the average estimate from all possible samples would lie. For example, approximately 95 percent of the intervals from 1.96 standard errors below the estimate to 1.96 standard errors above the estimate would include the average result of all possible samples.5 With a single survey estimate, the 95 percent confidence limit implies that if the same sample design were used over and over again, with confidence intervals determined each time from each sample, 95 percent of the time the confidence interval would enclose the true population value.

The number of survey estimates in the SDR for which standard errors might have been estimated was extremely large because of the number of variables measured, the number of subpopulations, and the values—totals, percentages, and medians—that were estimated. Direct calculation of standard error estimates from the raw data for each estimate was not possible because of time and cost limitations. Instead, a method was used for generalizing standard error values from a subset of survey estimates that characterize the population, allowing application to a wide variety of survey estimates.

4  

The weighted response rate is defined as the total returns (in-scope and out-of-scope) multiplied by their basic weights divided by those in the survey sample multiplied by their basic weights. Weighted response rates take into account the unequal probabilities of selection to the sample and indicate the potential for nonresponse bias in the survey estimates.

5  

Approximately 90 percent of the intervals from 1.64 standard errors above and below the estimate would include the average result of all possible samples; or, if more precision is required, approximately 99 percent of the intervals from 2.58 standard errors above and below the estimate would include the average result of all possible samples.

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

This method computes the variances associated with selected variables and uses these estimates to develop values of a and b parameters (regression coefficients) for use in generalized variance functions that estimate the standard errors associated with a broader range of totals and percentages. Base a and b parameters are shown in Table A-2. These parameters were used to generate tables of approximate standard errors shown as Tables A-3 through A-6. The use of these tables is described below, together with an alternative method for approximating the standard errors more directly.

Standard Errors of Estimated Totals

Tables A-3 and A-4 show approximate standard errors for the humanities doctoral population overall, for field groupings used in the report (e.g., history and philosophy), and for females by field. The standard errors shown in the tables were calculated using the appropriate values of a and b, along with the following formula for standard errors of totals:

where x is the total. Resulting values were rounded to the nearest multiple of 10. The illustration below shows how to use the tables to determine the standard errors of estimates shown in the report.

Illustration. The number of humanities Ph.D.s employed in the private for-profit sector is reported at 5,800. To determine the approximate standard error, one can use the values shown in Table A-3 for the estimated numbers of 5,000 and 10,000 in the "All Fields" column, or 320 and 450, respectively. Then, through linear interpolation, one can calculate 341 as the approximate standard error of the estimate of 5,800 as follows:

On the other hand, using the values of a and b for all humanities Ph.D.s from Table A-2 and Formula 1, one can also calculate the approximate standard error more directly:

To develop a 95 percent confidence interval around this estimate of 5,800, one would add and subtract from the estimate the standard error multiplied by 1.96. This means that the average estimate from all possible samples would be expected 95 times out of 100 to fall within the range of

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

This range of 5,118 to 6,482 represents the 95 percent confidence interval for the estimated number of 5,800.

Standard Errors of Estimated Percentages

Percentages are another type of estimate given throughout the report. The standard error of a percentage may be approximated using the following formula:

where x is the numerator of the percentage, y is the denominator of the percentage, p is the percentage (0 << p << 100), and b is from Table A-2. Tables of standard errors of estimated percentages were derived using this formula and are shown in Tables A-5 and A-6. Formula 2 may be used to calculate the standard errors of percentages not shown in the tables.

Illustration. Using the same example mentioned earlier but stated as a proportion, approximately 5.8 percent of all humanities doctorates were employed in the private for-profit sector. That is, of the 99,100 individuals who are employed, 5,800 were working in the private for-profit sector, or about 5.8 percent. Table A-5 shows the approximate standard error of a 5 percent characteristic on a base of 100,000 (the closest values) to be 0.3.

Alternatively, using the appropriate value of b from Table A-2 and Formula 2, the standard error of p may be determined as follows:

To develop a 95 percent confidence interval around this estimate of 5.8 percent, one would add and subtract from the estimate the standard error multiplied by 1.96. That is, the average estimate from all possible samples would be expected 95 times out of 100 to fall within the range of

The range of 5.11 to 6.49 represents the 95 percent confidence interval for the estimated percent of 5.8.

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

Limitations of the Standard Error Estimates

As mentioned, the standard error estimates provided in this report were derived from generalized functions on the basis of a limited set of characteristics (or survey estimates). Although this method provides good approximation of standard errors associated with most survey results, it may overstate the error associated with estimates drawn from strata with high sampling fractions. However, the only way to avoid this overstatement is to calculate the standard errors directly from the raw data, forgoing the practical, and more widely applicable, generalized method.

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

TABLE A-1 Response Rates by Summary Strata (Field, Cohort, and Gender), 1995

 

Sampling Frame

Survey Sample

In- Scope Returns

Out-of- Scope Returns

Total Returns

Weighted Response Rate (%)

Field of Doctorate

 

 

 

 

 

 

Art History

3,826

397

246

19

265

67.1

American History

7,536

569

391

6

397

69.8

Other History

17,043

1,070

639

55

694

65.2

Music

11,234

818

533

24

557

68.6

Speech/Theater

6,070

581

373

16

389

65.6

Philosophy

8,979

763

429

35

464

60.5

English/American Lang/Lit

29,624

2,042

1,263

61

1,324

65.6

French/Spanish Lang/Lit

9,295

773

443

24

467

60.9

Other Modern Lang/Lit

8,492

748

435

36

471

63.3

Classics

2,371

315

191

15

206

65.3

Other Humanities

10,573

753

450

30

480

64.2

Cohort

 

 

 

 

 

 

1985-1994 Doctorates

32,804

2,698

1,715

73

1,788

66.1

Pre-1985 Doctorates

82,239

6,131

3,678

248

3,926

64.7

Gender

 

 

 

 

 

 

Male/Unknown

73,364

5,521

3,292

203

3,495

64.0

Female

39,679

3,308

2,101

118

2,219

67.1

Total

115,043

8,829

5,393

321

5,714

65.1

NOTE: Out-of-scope sample cases are those learned to be deceased, living outside the United States, or over the age of 75. The weighted response rate is the total returns (in-scope and out-of-scope) multiplied by their basic weights divided by the survey sample multiplied by their basic weights.

SOURCE: National Research Council, Survey of Humanities Doctorates.

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

TABLE A-2 Listing of a and b Parameters (Select Groups in Humanities Fields), 1995

 

Gender

 

Years Since Doctorate

Field of Doctorate

Parameters

Total

Male

Female

 

5 or Less

6-15

16-25

Over 25

Total, Humanities

a

-0.0002

-0.0003

-0.0005

 

-0.001

-0.0007

-0.0005

-0.0016

 

b

22.0334

24.542

18.7954

 

19.5561

22.6625

20.9583

38.9508

History

a

-0.0011

-0.0016

-0.0024

 

-0.0031

-0.0035

-0.0017

-0.0137

 

b

27.5428

31.4577

12.5688

 

11.4072

20.1781

23.3711

86.5962

Art History

a

-0.0072

-0.0206

-0.0084

 

-0.0112

-0.0149

-0.0066

-0.0297

 

b

25.4867

30.2316

17.3578

 

7.792

19.7987

6.4701

18.3836

Music

a

-0.0013

-0.0011

-0.0092

 

-0.0041

-0.0025

-0.0018

-0.002

 

b

14.5382

8.6691

21.4679

 

11.9481

9.7411

8.1653

2.5097

Philosophy

a

-0.0016

-0.0028

-0.0067

 

-0.0037

-0.0062

-0.0005

-0.0014

 

b

14.448

19.0081

10.6307

 

4.0905

14.4724

1.5788

4.1062

Engl/Am Lang/Lit

a

-0.0007

-0.0011

-0.0008

 

-0.005

-0.0037

-0.0005

-0.0004

 

b

18.9447

18.1487

10.1054

 

20.8339

25.2734

4.602

2.9169

Classics

a

-0.0037

-0.0069

-0.0079

 

-0.0064

-0.021

-0.0837

-0.0065

 

b

8.3628

10.4816

5.4447

 

1.9178

12.3877

23.8383

3.6465

Modern Lang/Lit

a

-0.0007

-0.001

-0.0026

 

-0.0048

-0.0046

-0.0014

-0.0029

 

b

14.7339

11.9273

17.7044

 

16.2059

25.0407

12.9656

13.7691

Other Humanities

a

-0.0008

-0.0014

-0.0017

 

-0.0042

-0.0026

-0.0026

-0.0026

 

b

17.5967

19.0471

12.9988

 

18.4607

18.5882

17.3912

12.496

 

SOURCE: National Research Council, Survey of Humanities Doctorates.

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

TABLE A-3 Approximate Standard Error of Estimated Number of Humanities Doctorates, by Field, 1995

Estimated

All

 

Art

 

 

Engl/Am

 

Modern

Other

Number

Fields

History

History

Music

Philosophy

Lang/Lit

Classics

Lang/Lit

Humanities

50

30

40

40

30

30

30

20

30

30

100

50

50

50

40

40

40

30

40

40

200

70

70

70

50

50

60

40

50

60

500

100

120

100

80

80

100

60

80

90

700

120

140

120

100

100

110

60

100

110

1,000

150

160

140

120

110

140

70

120

130

2,500

230

250

140

170

160

210

-

180

200

5,000

320

330

 

200

180

280

-

240

260

10,000

450

410

-

-

-

350

-

280

310

25,000

650

-

-

-

-

190

-

-

-

50,000

780

-

-

-

-

-

-

-

-

75,000

730

-

-

-

-

-

-

-

-

100,000

450

-

-

-

-

-

-

-

-

TABLE A-4 Approximate Standard Error of Estimated Number of Female Humanities Doctorates, by Field, 1995

Estimated

All

 

Art

 

 

Engl/Am

 

Modern

Other

Number

Fields

History

History

Music

Philosophy

Lang/Lit

Classics

Lang/Lit

Humanities

50

30

20

30

30

20

20

20

30

30

100

40

40

40

50

30

30

20

40

40

200

60

50

60

60

40

40

30

60

50

500

100

80

80

90

60

70

30

90

80

700

110

90

90

100

60

80

-

110

90

1,000

140

100

90

110

60

100

-

120

110

2,500

210

130

-

-

-

140

-

170

150

5,000

290

-

-

-

-

170

-

150

150

10,000

370

-

-

-

-

150

-

-

-

25,000

400

-

-

-

-

-

-

-

-

 

SOURCE: National Research Council, Survey of Humanities Doctorates.

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×

TABLE A-5 Approximate Standard Errors of Estimated Percentages of Humanities Doctorates, 1995

Base Number

Estimated Percentages

 

 

 

 

 

 

of Percent

1 or 99

2 or 98

5 or 95

10 or 90

15 or 85

25 or 75

50

50

6.6

9.3

14.5

19.9

23.7

28.7

33.2

100

4.7

6.6

10.2

14.1

16.8

20.3

23.5

200

3.3

4.6

7.2

10.0

11.9

14.4

16.6

500

2.1

2.9

4.6

6.3

7.5

9.1

10.5

700

1.8

2.5

3.9

5.3

6.3

7.7

8.9

1,000

1.5

2.1

3.2

4.5

5.3

6.4

7.4

2,500

0.9

1.3

2.0

2.8

3.4

4.1

4.7

5,000

0.7

0.9

1.4

2.0

2.4

2.9

3.3

10,000

0.5

0.7

1.0

1.4

1.7

2.0

2.3

25,000

0.3

0.4

0.6

0.9

1.1

1.3

1.5

50,000

0.2

0.3

0.5

0.6

0.7

0.9

1.0

75,000

0.2

0.2

0.4

0.5

0.6

0.7

0.9

100,000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

 

SOURCE: National Research Council, Survey of Humanities Doctorates.

TABLE A-6 Approximate Standard Errors of Estimated Percentages of Female Humanities Doctorates, 1995

Base Number

Estimated Percentages

 

 

 

 

 

 

of Percent

1 or 99

2 or 98

5 or 95

10 or 90

15 or 85

25 or 75

50

50

6.1

8.6

13.4

18.4

21.9

26.5

30.7

100

4.3

6.1

9.4

13.0

15.5

18.8

21.7

200

3.1

4.3

6.7

9.2

10.9

13.3

15.3

500

1.9

2.7

4.2

5.8

6.9

8.4

9.7

700

1.6

2.3

3.6

4.9

5.9

7.1

8.2

1,000

1.4

1.9

3.0

4.1

4.9

5.9

6.9

2,500

0.9

1.2

1.9

2.6

3.1

3.8

4.3

5,000

0.6

0.9

1.3

1.8

2.2

2.7

3.1

10,000

0.4

0.6

0.9

1.3

1.5

1.9

2.2

25,000

0.3

0.4

0.6

0.8

1.0

1.2

1.4

 

SOURCE: National Research Council, Survey of Humanities Doctorates.

Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
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Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
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Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×
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Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×
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Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
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Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×
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Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
×
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Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
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Page 71
Suggested Citation:"APPENDIX A." National Research Council. 1997. Humanities Doctorates in the United States: 1995 Profile. Washington, DC: The National Academies Press. doi: 10.17226/5840.
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Page 72
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