The two surveys are sampled and conducted independently. Expenditure estimates are made independently from the two surveys for various purchased items (goods and services). The concept of having two distinct surveys is fairly simple. The Interview survey was “designed to collect data on the types of expenditures respondents can be expected to recall for a period of 3 months or longer. In general, expenditures reported in the Interview Survey are either relatively large, such as for property, automobiles, or major appliances, or occur on a fairly regular basis, such as for rent, utility bills, or insurance premiums” (Bureau of Labor Statistics, 2008, p. 2). On the other hand, the Diary survey was designed “to obtain expenditure data on small, frequently purchased items, which are normally difficult to recall. These items include food and beverage expenditures, at home and in eating places; housekeeping supplies and services; nonprescription drugs; and personal care products and services” (Bureau of Labor Statistics, 2008, p. 3).
In reality, there is considerable overlap in expense items collected on the surveys, increasing the total response burden. The Interview survey, reaching beyond its original design, also collects information on small, frequently purchased items that may be difficult to recall. For example, it collects expenses for prescription medication, fresh flowers, sewing notions, and the full range of clothing and shoes. It also collects average monthly cost of gasoline and the average weekly cost of buying food in a grocery store. Similarly, the Diary survey, with its open listing sheets, collects many larger items identified as appropriate for the Interview survey. For example, it collects information on the purchase of dishwashers, china, jewelry, and vehicles (Bureau of Labor Statistics, 2011b,c).
The final Bureau of Labor Statistics (BLS) estimate presented in the Consumer Expenditure Survey Integrated Tables for any particular item is based on an estimate from one or the other of these surveys. Creech and Steinberg (2011) describe how the survey source is selected for each published item, and Bureau of Labor Statistics (2009a) provides examples of which source was used for different items for the Consumer Expenditure Survey Integrated Tables for 2009. Some choices are fairly obvious. New refrigerators and living room chairs were estimated from the Interview survey. The Diary survey was used to estimate expenditures for “wine consumed at home” and “lunch at a fast food restaurant.” However, the choice is not always intuitive. For example, bedroom linens were estimated from the Diary survey, while curtains and draperies were estimated from the Interview survey. Under the grouping of “housewares,” silver service pieces were estimated from the Diary survey and other service pieces from the Interview survey. Luggage was estimated from the Diary survey, while smoke alarms were estimated from the Interview survey.