{"id":250,"date":"2012-07-10T21:11:16","date_gmt":"2012-07-10T14:11:16","guid":{"rendered":"https:\/\/blogs.ubc.ca\/fishonfarms\/?p=250"},"modified":"2012-07-11T17:41:57","modified_gmt":"2012-07-11T10:41:57","slug":"sampling-methodology","status":"publish","type":"post","link":"https:\/\/blogs.ubc.ca\/fishonfarms\/2012\/07\/10\/sampling-methodology\/","title":{"rendered":"Sampling Methodology"},"content":{"rendered":"

O<\/strong><\/span>ne of the issues that we have encountered during the first week of baseline is that some of the houses that were originally selected to be part of the study are no longer eligible.<\/p>\n

In some instances, the mother went to Phnom Penh or the Thai border to seek work. The purpose of our study is to show the effects of HFP<\/strong><\/span> and aquaculture<\/strong><\/span> on household food security and the nutritional status of women and children. As such, the woman of the house must be present year-round to reap the benefits of these interventions<\/strong><\/span>. By migrating for work, a woman\u2019s nutritional status at the end of the study would not reflect the addition of HFP and aquaculture in her life. Therefore, her household is now ineligible<\/strong><\/span> for participation in FoF.<\/p>\n

In other instances, when we arrived at the house we found out that the children were over the cutoff age of 5<\/strong><\/span> years old. This happened for a variety of reasons: some women couldn\u2019t accurately remember the date of birth (the Khmer calendar is different from ours), the selection team didn\u2019t check the proper documents to verify the child\u2019s age, or the age on the village chief\u2019s list was incorrect. It\u2019s also possible that some women provided false ages for their children because they wanted to be part of a study that provides the expensive inputs needed for HFP and aquaculture.<\/p>\n

In any event, our team has been scrambling to fill their spots so that we have the right sample size for the study. We\u2019ve accounted for a possible 15%<\/strong><\/span> of houses lost due to follow up, but we want to start with the biggest sample possible to minimize that loss given how expensive and time-consuming the project is.\u00a0 <\/span><\/p>\n

This seems like a good opportunity to discuss the methodology<\/strong><\/span> behind household selection. In an earlier post (Household Selection<\/span><\/a><\/span>), I briefly outlined the criteria<\/span><\/strong> that households needed to meet to be part of FoF. However, meeting the selection criteria does not mean a household is automatically enrolled in FoF. This is, after all, a scientific experiment<\/span>; certain research principles<\/strong><\/span> must be upheld.<\/p>\n

In order for this to be a valid<\/strong><\/span> and reliable<\/strong><\/span> scientific experiment, we need our sample<\/strong><\/span> to represent our target population<\/strong><\/span> as closely as possible while eliminating any potential biases<\/strong><\/span> or confounders<\/strong><\/span>. We achieve this by picking the proper sampling method. FoF is using a multi-stage sampling strategy<\/strong><\/span>. The first stage is cluster sampling<\/strong><\/span>, which is a form of probability sampling that examines naturally occurring groups such as villages. The second stage is systematic sampling<\/strong><\/span>, which is used to select the houses in the villages by picking a random point to start (eg the fourth house on the list) and continuing through the list in a systematic fashion (eg every fourth house on the list). A few key definitions<\/strong> are needed at this point:<\/p>\n