Selection of participants

EHES Training Material

Who can be invited?

• In principle, every person age 25 to 64 years living in the country is eligible

• Temporary visitors are not included in the survey

How are people selected?

Stage 1• The country is divided into examination areas• A number of these are selected randomly

An example country

How are people selected?

Stage 2• Within each selected examination area we select

people from the population register also randomly

An example country

How are people selected?

Stage 2• Within each selected examination area we select

households from a local household list also randomly

An example country

How are people selected?

Stage 3• Within each selected household we select all

household members

An example country

Stage 3• Within each selected household we select

1 person

How are people selected?

An example country

What is random selection?

• Selecting a person, household, dwelling or area randomly means that they are selected entirely by chance

• We can calculate how likely someone is to be selected.

• We can not calculate if they actually will be selected – this is the random part.

Why random selection?

• To estimate the health of the population we need to know everyone’s chances of being selected/invited

• This is only possible with random selection• Replacing someone who does not want to or can

not participate with someone else means we no longer have a random sample and can not estimate health figures accurately from the data

Sample

Biased samples

• A sample is biased if it does not reflect the population and will tend to give the wrong result

• Biased samples can result from:• Samples that are not randomly taken from the

population• Low response rates among certain groups of the sample

(eg. people who are not well)

Biased sample

Sample

Population Population

Representative sample Biased sample

Questions and answers

The following slides are frequently asked questions that may come from the survey participants and which fieldwork staff should be able to answer

“Why was I selected for the survey?”

• We have drawn a random sample of people from all over the country using the population register. It is share chance that you have been selected.

“How many people have you invited to participate in this

survey?”

• We have invited 4000 people from all over the country.

”Why do you select participants at random?”

• We can’t ask everyone to participate so we invite a small fraction of the population. It is important for this group to be selected randomly so they reflect the population as a whole.

”How can you learn about the general health for the entire

country by only examining a few people?”

• It’s not feasible to interview and examine everybody in the country. Instead we randomly select a group to represent the country. This is enough for us to gauge an accurate picture of the country’s health.

“What do I get in return for participating?”

• You will receive (mention incentive and) a free health check-up

• Additionally, the information is important for research and health monitoring purposes, which are needed to plan health services and health promotion activities for future generations

”My husband/friend is participating and I would like to

too. Can I turn up?”• It’s great that you are interested in the survey but

unfortunately we can’t include you this time as we only have the resources to survey those we selected randomly to participate.

• It’s great that you are interested in the survey and you are welcome to come on (date/time). Unfortunately you won’t be included in our official data but you can still join in and receive your test results.

”I don’t mind answering some health questions but I’m not

prepared to give a blood sample so I don’t think I should

participate at all.”

• It is very important for the survey that you participate in any way you can. We can start with the questionnaire and measurements and if you want to stop there that’s fine. You can stop at any time during the survey.

Acknowledgements

• Slides• Susie Jentoft and Johan Heldal