In general, it is good practice to implement a clear protocol for when to contact respondents and how many call attempts to make. Ensuring variation in the day of the week and time of day that call attempts are made can help in reaching a respondent at a day and time where they are available to answer and be interviewed. Providing small incentives to respondents may also help to ensure cooperation and reduce nonresponse, though the incentive has to be carefully calibrated to not contaminate responses from the respondent. One critical principle is to make all efforts to limit respondent fatigue through keeping the interview length manageable and the questions asked straightforward and (wherever possible) avoiding asking sensitive or uncomfortable questions. These and other approaches are further discussed in Section 4.2 below.

# 3.2. Respondent selection

The choice of respondent is a key survey design decision for any survey. The survey objectives, and, in particular, the desired unit of observation, inform this decision. Survey respondents need to have knowledge of the issues in which the survey is interested, and the choice of survey respondent should reflect that. This requirement is not unique to phone surveys, but it can be substantially more difficult in practice to reach a desired respondent through a phone survey. Phone ownership within the household and among the general population is often skewed towards household heads, men, and older members (Brubaker et al., 2021), so interviewers may need to ask for phone owners to pass the phone along to the desired respondent, provided they are available at the time of the call (or schedule an appointment for another call). If survey questions are sensitive and require respondent privacy, phone surveys may not be a suitable tool (Section 4.2). Interviewing more than one household member, for instance all plot managers in an agricultural household, increases the response and time burden on the household and may carry the risk of greater attrition.

When it comes to collecting individual-level data, for instance on nutrition or individual food security, selecting a sample of individuals representative of the population of individuals is preferrable. For phone surveys, the challenge for individual-level data collection again comes down to the pattern of phone ownership whereby some segments of the population are less likely to own or have access to a mobile phone and thus will not be adequately represented in phone survey samples. Such issues can be magnified in recontact surveys that use the household (rather than the individual) as their unit of observation. Brubaker et al. (2021) show that the main respondents in High Frequency Phone Surveys on COVID-19 in four African countries are predominantly household heads, and also better educated and more likely to own a non-farm enterprise than adults overall, so that individual-level estimates are not representative of the population at large. In turn, the scope for a ‘proxy respondent’ answering on behalf of other household members depends critically on the kind of information collected and has been linked to measurement error (Kilic et al., 2020). For these reasons, a randomized (probability-based) respondent selection protocol—whereby a targeted respondent within the roster of eligible household members is randomly selected for interview—is the preferred strategy to achieve unbiased individual-level data.

# 3.3. Post-survey adjustments

When mobile phone ownership is not complete, the phone-owning population and the general population that the phone survey seeks to represent are different, often resulting in coverage bias. Similarly, if nonresponse is systematic, then the responding sample will not be representative of the population of interest. There are a range of post-survey reweighting techniques that can be used to attempt to counteract these differences and improve the representativeness of the estimates obtained (Valliant et 12