Category Archives: Sampling

Get the answer you Need. Not necessarily the one you want

Sampling Bias

Why does every little kid, who doesn’t get the answer they want from Mom, then go and ask Dad or Grandma or Auntie hoping to get a different response? Because kids know that the answer you get to your question depends on who you ask!!

In research it’s the same. Sometimes the answer you get depends on who you ask. Thus, it’s not only important to write out a well-formulated research question, but you need to ask the right persons! Unlike little children, however, you need to ask a representative sample that will give you an accurate scientific answer–not just an answer that matches your preconceived notions of the answer you want. The answer you need is not necessarily the one you want…or expect.

What research sample will give you a more accurate, scientific answer? It’s a sample that represents the population of interest. NO sampling method guarantees a representative sample from the population, but some sort of random selection of participants from the population is likely to be best. Sometimes, however, it’s not possible or affordable to do random sampling and so researchers pick their sample using a different sampling method.

A concise 5 minute summary of some sampling methods is here: https://www.youtube.com/watch?v=be9e-Q-jC-0

Remember that the BEST samples are REPRESENTATIVE. That may or may not mean they are randomly selected.

Whatever sampling method is used, be clear yourself about the strengths and weaknesses of that sampling method. Report your sampling method in the procedures section of your final report. That way your readers can tell whether you asked subjects who were the equivalent of “Mom,” “Dad,” or both.

“Who’s in?” Sampling Made Simple

So many great resources are out there at your fingertips.   In the next few blogs, I’ll point you to some of them.Heart Books

If you have not yet discovered the hidden jewel of the tab, “Research Made Simple,” on the Evidence-Based Nursing website at http://ebn.bmj.com/ , you should check out its full text pdf articles. An especially nice resource if you are in graduate school or maybe even a BSN course on research.

Here is a sample of one from June 9, 2014 of the population of articles at EBN.

ball sample“Sample selection is a key factor in research design and can determine whether research questions will be answered. …It is critical to take the time to clearly identify the population of interest for the specific research question. Nursing researchers are usually interested in answering questions about very specific patient populations which can span an incredible array of possibilities applying to international, national, local and organisational contexts. Research populations closely reflect nursing specialties, some of which are gender (eg, pregnant women) and age specific (eg, adolescent diabetes). It is rarely feasible to conduct a study that reaches every patient in the population of interest, therefore a subset or sample of that population is selected for study.” (para 1-2 Retrieved from http://ebn.bmj.com/content/17/2/32.short?g=w_ebn_research_tab ) [note: I added the bold]

I invite you to use the comments section to post URLs for other great sites!

Critical Thinking: Read the previous blog on representatives & then QUESTIONdecide for yourself whether this single EBN article on sampling is representative of clear & simple explanations. Samples can be things, not just people. What questions should you ask me about my sampling procedure?

For More: Check out tab “Research Made Simple,” on the Evidence-Based Nursing website at http://ebn.bmj.com/ .   OR try this <5 minute simple sampling youtube video  https://www.youtube.com/watch?v=Gs-gLeYuDZw

“That is so random!” But is it Representative?

What makes a good sample in research?  One thing.  And it isn’t random selection.  (Surprised?)

Portrait of a diversity Mixed Age and Multi-generation Family embracing and standing together. Isolated on white background. [url=http://www.istockphoto.com/search/lightbox/9786738][img]http://dl.dropbox.com/u/40117171/group.jpg[/img][/url]It is representativeness.  No matter how the sample was picked, it must be representative of all those in the larger population, if the researcher wants to say anything about anyone who wasn’t in the study.  Now, of course, it is true that random selection is more likely to give you a representative sample, but it is no guarantee.  Only likely.

What is random sampling?  It is when every member of the larger population has an equal chance of being selected for the study sample.  Example? Drawing names out of a hat.  It is well-accepted practice to generalize research results from a random sample to others like those being studied (assuming that all other aspects of the study are strong).

In contrast a convenience (or nonprobability) sample is when some people are more likely to be chosen to be in the study than others.  You shouldn’t generalize the results of these studies because the samples may Not represent others.

Example of when random sampling doesn’t work: Let’s say you have a mixture of red, green, & yellow apples, and you select a sample that has only yellow apples.  (The red & green ones are going to be offended!–They’re left out.)  You now have a sample that is biased in favor of yellow apples!   Your sample does Not represent the larger population of apples…even if you used random methods to get it.  If you want to apply the study to red & green & yellow apples…well….you must get some of them in your sample, too. The yellow apples might not be at all like the other types and studying just yellow might mislead you into thinking something about the red & green ones that isn’t true!   Of course you could study all the millions of apples in the world and exclude none, but that would be pretty cumbersome and expensive.   So, it’s better to go for a representative sample!

When else doesn’t random sampling create a representative sample?   If I am doing historical research, say on the Nursing Department at California State University/Northridge, then I want to hand pick the specific RNs by name who were in charge of the Department from the beginning.  Randomly selecting nurses from those who worked at the University won’t represent those leaders.

QUESTIONCritical Thinking:  Take a quick look at the linked abstracts. How were the samples selected?  How representative are the samples of a larger population of interest?  Could you generalize the results to other people, and if so to whom?

Want more information on sampline? Check this out.  It takes < 5 minutes:

https://www.youtube.com/watch?feature=endscreen&v=be9e-Q-jC-0&NR=1