efsa LEARNING PROGRAMME Module 4 - Session 4.5a Sampling

efsa LEARNING PROGRAMME Learning Objectives By the end of this session, you should be able to:  Describe the different types of sampling methods and their strengths and weaknesses  Select an appropriate sampling method  Be aware of how sampling methods can be linked to the collection, analysis, and interpretation of data  Know the sample designs that can be used for an EFSA and Rapid Rural Assessment

efsa LEARNING PROGRAMME Sampling Overview Sampling is the process of selecting a small number of people from a larger group of people The goal of sampling is to estimate something in a larger group To estimate something in a larger group, the smaller group must be representative of the larger group

efsa LEARNING PROGRAMME Sampling Overview Sampling can be a complicated process and can be simplified by understanding a few key elements Non-proportional sampling Accuracy Random selection Precision Representative Bias

efsa LEARNING PROGRAMME Sampling Overview Sampling can be a complicated process and can be simplified by understanding a few key elements Sampling is done because it is cheaper and quicker than questioning everyone in the area and will give you accurate information if done right there are 600,000 women in the province ten teams can interview 60 women a day interviewing 200 women will take 3.3 days interviewing 600,000 women will take 2.7 years

efsa LEARNING PROGRAMME Sampling Overview Sampling can be a complicated process and can be simplified by understanding a few key elements Sampling is done because it is cheaper and quicker than questioning everyone in the area The challenge in sampling is to select the right group of people to interview accurately interpret the information collected from the smaller sampled group to the larger target population

efsa LEARNING PROGRAMME First, the people… Target Population The people that you want to provide assistance Sample Population The group of people that you want to interview RespondentThe people that you interview Analysis unitWho the survey data refer to

efsa LEARNING PROGRAMME then a few key elements… Random: Everyone has an equal chance of being interviewed Bias: Some people have greater chance of being interviewed than others Representative: The information from the sample is similar to the information if everybody in the target population was interviewed

efsa LEARNING PROGRAMME then the numbers… Precision Ability to get the same results if you do the survey again using same questionnaire and sampling design Accuracy Ability to get results close to the truth ValidityAbility to measure what you want to measure

efsa LEARNING PROGRAMME Precision and Accuracy Not precise Not accurate Not precise Accurate Precise Not accurate Accurate Precise

efsa LEARNING PROGRAMME Pop Quiz Representative or bias sample?

efsa LEARNING PROGRAMME Representative or biased sample? A survey of malnutrition in children uses radio announcements to ask women to bring their child to the health clinic on Tuesday morning.

efsa LEARNING PROGRAMME biased…  Mothers who use the clinic may not be representative of all mothers in the population  Only people with a radio can hear about the survey  Mothers must go to the clinic on Tuesday  Mothers who use the clinic may not be representative of all mothers in the population  Only people with a radio can hear about the survey  Mothers must go to the clinic on Tuesday

efsa LEARNING PROGRAMME All the houses in a village is numbered. You randomly select 1% of the houses and write down the house number that has been selected. You go to the house number and interview the occupants. Representative or biased sample?

efsa LEARNING PROGRAMME representative…  You had a list of all the houses in the village  Every house had an equal chance of being selected  You were able to go the houses that were selected  This is an example of simple random selection  You had a list of all the houses in the village  Every house had an equal chance of being selected  You were able to go the houses that were selected  This is an example of simple random selection

efsa LEARNING PROGRAMME An NGO website asks online visitors to answer questions on whether NGOs appropriately use food assistance in emergencies Representative or biased sample?

efsa LEARNING PROGRAMME biased…  People who visit the NGO home page may not be representative of the humanitarian community  People with strong opinions about the subject are more likely to participate  This is an example of self-selection bias where participants choose to be sampled  People who visit the NGO home page may not be representative of the humanitarian community  People with strong opinions about the subject are more likely to participate  This is an example of self-selection bias where participants choose to be sampled

efsa LEARNING PROGRAMME Errors in Sampling Sampling Not representative of target population Random selection of respondents Too few participants Recruit more participants Measurement Confusing questions Pilot questions Interviewer suggests responses Train interviewers Response People refuse to be interviewed Train interviewers People give response they think you want Train interviewers

efsa LEARNING PROGRAMME and the two approaches… ProbabilityRandom methods decide who is selected and the chance of a person being selected is known Non- probability Subjective judgment is used to select the sample and you do not know the chance of a person being selected

efsa LEARNING PROGRAMME Examples of probability sampling You have a list of 35,000 people registered in a refugee camp. You sample every 350 th person on the list. The chance of being selected is 350/35,000 or 1 in 1000.

efsa LEARNING PROGRAMME Examples of probability sampling You have the names of the 45 villages in a province and write the names of each village on a piece of paper. You put all the pieces of paper in a glass jar, mix the pieces of paper and pick out 5 pieces. The chance of a village being selected is 5 out of 45 or 1 in 9.

efsa LEARNING PROGRAMME Examples of non-probability sampling You go to the village square and ask people in the market about the kind of food they have at their house.

efsa LEARNING PROGRAMME Examples of non-probability sampling You go to a village and speak to the key informants about food security and availability

efsa LEARNING PROGRAMME When to Use Probability and Non-probability Sampling  Probability sampling  Number or percent of people with a characteristic  Assess small changes when monitoring or evaluating a program  Nutritional or health surveys use probability sampling  Non-probability sampling  Some information is more important than an accurate and precise number  Pilot testing  Rapid appraisal methods often use non-probability sampling

efsa LEARNING PROGRAMME The big difference… Probability sampling – a lot of effort is taken to select the person to interview because that person represents many other people. That process must be replicated for each person that is interviewed. Non-probability sampling – there is less restrictions on selecting the person to interview.

efsa LEARNING PROGRAMME so…why bother with probability sampling? Sample is representative of a larger population Results can be generalized Minimizes bias for selecting people to be interviewed

efsa LEARNING PROGRAMME so…why bother with non-probability sampling? Easy and fast recruitment Explore a problem and some basic idea of a solution Provide insight and comprehension of a situation Good at probing below the surface for affective drives and subconscious motivations May be only realistic option in an emergency

efsa LEARNING PROGRAMME Summary Sampling gathers data on a small group of people to help understand what is going on in a larger group Probability and non-probability sampling are the two major sampling designs The challenge in sampling is to 1) select the right group of people to interview and 2) accurately interpret the information collected from the smaller sampled group to the larger target population