indicator, a survey usually needs at least 50 consumption items. Further, food items need to include own production and gifts in addition to purchases. As a result, collecting full-fledged consumption data requires one hour or more. However, according to L2L, it appears difficult to continue telephone interviews for more than 15 minutes. This would be a reason most telephone surveys do not collect consumption or income data.

## (iii) Response bias

Given that collection of consumption or income data via telephone interviews is difficult, S2S is very attractive since it requires only 10 to 15 simple questions. But, there might be a problem if responses to telephone interviews differ significantly than those to face-to-face interviews and the response bias causes a bias in poverty estimation using an S2S approach. An S2S formula is usually developed in a household survey that was collected via face-to-face interviews. But, if a household responded to telephone surveys differently than face-to-face interviews, projections of household expenditure or poverty rates would be biased.

Some studies indeed show evidence of such response bias. L2L conducted comprehensive and well-designed experiments on response bias due to interview modes. It compared four different interview modes: a face-to-face interview, IVR, SMS, and Computer Assisted Telephone Interview (CATI). The study found that responses to CATI are consistent with those to face-toface interviews, while responses to IVR or SMS are significantly different from those to face-toface interviews. Croke et al. (2012) also found similar results. Dillman et al. (2009) compared CATI with IVR, web-based data collection, and data collection via mail. They found that respondents to CATI and IVR tend to choose more optimistic or socially desirable answers than those to the web-based data collection or mailing. In terms of comparison between CATI and IVR, there is no clear tendency but they found responses to CATI are significantly different from those to IVR. Mu (1998) found respondents to IVR are less likely to select “10” and more likely to select “9” than those to CATI. This might be because respondents feel it is more cumbersome to type two-digit numbers in a telephone keyboard. Tourangeau et al. (2002) found that answers to IVR are slightly more positive than those to CATI.

# **3 A New Approach for Telephone Data Collection**

Based on findings of the previous studies, we propose the following approach for telephone surveys to estimate reliable poverty and inequality statistics that are representative at a level we are interested in:

1. Develop a formula using the latest household survey that includes both household expenditures and its correlates;

2. Select households so that the sample becomes representative at a level of interest, and conduct a baseline survey to collect telephone numbers from these households. If they do not have any telephone, provide them a cell phone; 3. Inform select households that this survey is part of official household surveys; 4. Operators call these households, ask questions on correlates of household expenditure, and input the responses to computers using CATI; 5. Impute household expenditures from the correlates and estimate poverty and inequality statistics.

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