TABLE 7.1 Distributions of the Number of Flight Legs Flown During the 60-Day Recall Period, for the Years 2002-2004

No. of Legs Flown

2002

2003

2004

61-100

944

907

805

101-150

267

209

175

151-200

66

58

42

201-250

13

14

6

251-300

14

9

7

301-350

2

4

2

351-400

2

8

2

401-450

3

1

0

451-500

2

3

2

501-550

1

1

0

551-600

1

0

0

601-650

1

0

0

NOTE: For Category 5: greater than 60 legs.

TABLE 7.2 Distributions of the Number of Hours Flown During the 60-Day Recall Period, for the Years 2002-2004

No. of Legs Flown

2002

2003

2004

151-200

713

716

802

201-250

234

32

18

251-300

19

6

2

301-350

2

2

1

351-400

2

3

1

401-450

2

0

1

451-500

2

0

0

501-550

1

0

0

551-600

1

0

0

NOTE: For Category 6: greater than 150 hours.

The committee’s analysis showed that the problem with anomalous values is common across many event types. Table 7.3 provides selected examples. For the event AH4 (“inadvertently landed without clearance at an airport with an active control tower”), a total of 541 events were reported by 161 pilots. Of these, 4 pilots reported 10, 20, 30, and 303 events (the last response corresponding to a pilot who flew between 46 and 70 hours and fewer than 14 legs during the recall period). These 4 pilots accounted for 363 (67%) of the 541 events reported. Table 7.3 shows several other examples with unusually high numbers of events reported. If the instances of such anomalies were limited to only a few event types, one might be able to investigate them in greater detail. Unfortunately, however, the problem was extensive, with one, and often several, implausible values for many event types.

There are at least two possible reasons for these anomalous values: (1) the pilots gave erroneous answers, or (2) errors were made during data entry. A verification question for high values for hours flown was included in the questionnaire, but the committee does not know if other data-audit procedures were in place to flag implausible values reported by the respondents or entered into the database.

7.1.2
Rounding

Another characteristic common in the survey data was the rounding of the responses (raw data) by the respondents (pilots). A disproportionate number of observations were rounded, either to have zero or 5 as the last



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement