Find a small jar or container. Take a piece of plain, A4 paper and cut it into 100 squares of the same size. On each of the squares draw either a red, blue, black or yellow cross (so that you have 25 squares with a red cross, 25 with a blue cross etc.). Fold each of the squares so that the coloured cross cannot be seen and place them in the jar. Draw out 20 squares.

Do you have an equal number of squares with a red, blue, black and yellow cross?

If not, how representative of your population is your sample?

What might this mean for the findings of a research study using this sampling strategy? 

Take the container and 24 of the squares with a red cross and 24 of the squares with a blue cross that were used in website activity 1. Write M (for male) against 12 of the blue and 12 of the red crosses. Write F (for female) against 12 of the blue and 12 of the red crosses. You should now have 12 blue crosses with an M, 12 blue crosses with an F, 12 red crosses with an M and 12 red crosses with an F.

Place the 24 squares with an M in the container and draw out 10. Then place the 24 squares with an F in the container and draw out 10.

How representative of your population is your sample?

Whilst you will have an equal number of male and females in your sample, you may still have an equal number of red and blue squares. How could a researcher avoid this problem occurring? 

For further insight to snowball sampling read:

Sadler, G.R. Lee, H-C. Lim, R.S-H. and Fullerton, J. (2010) Recruitment of hard-to-reach population sib-groups via adaptations of the snowball sampling strategy, Nursing and Health Sciences 12(3): 369–374.

For further insight to theoretical sampling read Chapter 7, ‘Theoretical sampling’ in:

Corbin, J. and Strauss, A. (2015) Basics of Qualitative Research, 4th edn. Los Angeles: Sage Publications.

We have covered the most commonly used sampling strategies in this book. However, other sampling strategies may be used; therefore look up and make notes on the following sampling strategies which you may see used in health care research:

• Multi-stage sampling
• Dimensional sampling
• Proportionate stratified sampling
• Disproportionate stratified sampling
• Volunteer sampling

Read the following two papers. One is a quantitative and the other a qualitative study:

Fenstermacher, K.H. (2014) Enduring to gain perspective: A grounded theory study of the experience of perinatal bereavement in Black adolescents, Research in Nursing and Health 37(2): 135–143.

Law, S.M. Dunn, O.M. Wallace, L.M. and Inch, S.A. (2007) Breastfeeding Best Start study: training midwives in a ‘hands off’ positioning and attachment intervention, Maternal and Child Nutrition 3(3): 194–205.

As you read the papers consider the following:

• How are the sample sizes explained and justified? Do you think an appropriate rationale is given?
• What were the participant inclusion and exclusion criteria?
• Did the sampling strategies enable the researchers to achieve the fundamental principles of the research method used?

What are the pros and cons of the following ways in which contact could be made with potential participants?

• Face-to-face discussion
• Letter
• Telephone call
• Email
• Word-of-mouth (snowball sampling)
• Posters displayed in venues where potential participants will see them
• Notices on websites or in journals

The class of 2016 have been considering the following research questions and hypotheses and are trying to decide which sampling strategy to use and potential participant inclusion and exclusion criteria. Review the following:

An RCT – Preterm babies who are exclusively fed their mother’s breast milk gain weight faster than preterm babies who receive formula milk.

A phenomenological study – What is the lived experience of teenagers with chronic renal failure?

A survey – What are midwives’ experiences of water-birth?

• Identify which sampling strategy (probability or non-probability) should be used and give a rationale for your decision.
• Select the most appropriate specific sampling strategy for the studies, giving a rationale for your decision and your exclusion of other options.
• Devise potential participant inclusion and exclusion criteria for the studies.