global questions and answers to results from detailed specific questions:
Overview and summary
How do the dollar amounts collected from global questions that ask respondents to estimate their usual spending for a certain time period compare to answers gathered by specific questions? Are they higher or lower, or similar? Research comparing 20 years of reports of itemized expenditures on Food at home and Food away from home from the Consumer Expenditure Survey (CE) Diary instrument with the responses from the global food questions from the Interview Survey show that it depends. Using detailed prepublication data from 1998 to 2017, the aggregate dollars reported in the Diary Survey for specific food purchases for eating at home are always lower than the global estimates from the Interview Survey. The Diary to Interview ratio for this group of Food at home items has consistently ranged between 76 percent and 82 percent.
Conversely, the expenditures reported for eating at restaurants and away from home collected in the Diary Survey are always higher than the global estimates from the Interview Survey. The range is greater, from 166 percent higher to 189 percent higher between 1998 and 2006, and then from 104 percent to 128 percent higher between 2007 and 2017. There was a change in how the global question in the Interview Survey was asked in 2007 which caused a shift in the ratios.
Results and analysis
Table 1 shows how the reported amounts for food from the Diary Survey compare with global estimates for food expenditures from the Interview Survey for each year. See the Methodology section below for how the categories were adjusted.
The Food at home Diary/Interview Ratio column shows that itemized purchases in the Diary Survey are consistently lower than the global estimates from the Interview Survey, ranging from 76 to 81 percent.
The Food away from home Diary/Interview Ratio column shows that respondents in the Diary Survey always report higher spending on itemized meals away from home in their week of diary record keeping, compared to the global estimates by respondents in the Interview Survey asked to provide either a monthly or weekly estimate.
The last column in the table shows the ratio of total aggregate dollars spent for Food at home plus Food away from home from the Diary Survey compared to Food at home plus Food away from home from the Interview Survey for comparable categories. Starting in 2007, the Diary Survey combined amounts for both food categories have been between 86 and 94 percent of the Interview Survey amounts.
The table shows that using the combined global estimates for food from the Interview Survey instead of the reported amounts from the Diary Survey would exceed the reported dollars spent by U.S. Diary households on food. The positive dollar difference for Interview groceries is larger than the negative dollar difference for restaurant meals, and the differences do not cancel each other out.
Why are expenditures for itemized Food at home purchases from the Diary Survey always lower than global Interview Survey responses? There are two suggested causes.
Why are itemized food away from home purchases always higher than global estimates? And what caused the change in the ratios from 2006 to 2007? There are two key findings.
Conclusions and impact for data users
The results of this comparison are important for researchers who select data from only one survey, rather than using the integrated results from the Diary and Interview Surveys that are used in the published CE tables and as cost weights used by the Consumer Price Index.
These results suggest that respondents do not report the same dollar amounts when asked for global estimates compared to itemizing expenditures.
Researchers should be aware of these differences when using the CE data. Analysis that only uses food expenditures reported in the Interview Survey will have lower totals than the Diary Survey amount spent at restaurants, and will have higher totals than the Diary reported amounts spent on food for home consumption.
Methodology and data used
The data used in this report come from the annual detailed prepublication tables for Integrated data, Interview Survey only data, and Diary Survey only data. In order to make direct comparisons between the Diary and Interview Surveys dollar amounts, the following adjustments were made. Most of the published total food expenditures in the annual tables come from the Diary Survey; however a few expenditures in the integrated total are sourced from the Interview Survey. For the Food at home comparisons used in this research paper, the only Food at Home expenditure that comes from the Interview Survey -- food for home consumption purchased on trips -- was subtracted from the total before calculating the ratios. For calculating the Diary-to-Interview ratios for Food away from home, the expenditure amounts for integrated categories that come from the Interview Survey, i.e., board, catered affairs, meals during out of town trips, school lunches, and meals as pay, were subtracted from the total.
For a list of the detailed prepublication categories for all expenditures including food and their selected sources from the Interview or Diary Survey, see the annual "All consumer units: Annual detailed expenditure means, standard errors, coefficients of variation, and weekly (D) or quarterly (I) percent reporting" table.
As a historical note, these same ratios in this 1998 through 2017 data comparison have been observed since the CE began collecting data on a continuous basis. The first published look at the 1980-81 Interview Survey results included these paragraphs:
For food-at-home expenditures, respondents in the Interview survey are asked a "global" question, that is, to estimate the number of trips they made to the grocery store for the 3-month period prior to the Interview survey, the average expenditures per trip, and how much of the expenditure was for nonfood items, which is then subtracted from the total. Food expenditures at specialty and convenience stores are added to give an estimate of food at home. In the Diary survey, respondents keep an itemized record of daily expenditures on food and other items. Food-at-home estimates from the Interview survey were about 22 percent to 23 percent higher than the Diary survey estimates in 1980 and 1981.
The opposite result was obtained for expenditures for food away from home, which are also collected with a global question in the Interview survey. Expenditures for food away from home (including trips) in the Interview survey were about 4 percent lower than the Diary survey estimates in 1980 and about 5 percent lower in 1981.
Source: Consumer Expenditure Survey: Interview Survey, 1980-81, US Department of Labor Bureau of Labor Statistics April 1985 Bulletin 2225, page 8.
Last Modified Date: April 4, 2019