If you’ve ever looked at results from a public opinion survey or political poll, you’ve no doubt seen the margin or error noted alongside the findings. Usually the note will read something like margin of error = plus or minus x%, CL 95%.
So what the heck does that mean?
Well, the first part basically tells you how close the results from your selected sample are compared to what you'd find if you surveyed the entire population. The expression of “plus or minus x%” tells you that the percentages of given responses might be a bit higher or lower “in reality” (i.e., if you surveyed absolutely everyone).
Generalizability to the larger population is also described by an associated measure called a confidence level (CL). This term describes how confident you can be that your results are not due to chance alone. A confidence level is normally set at either 90%, 95% or 99% (95% has become standard).
Let’s use an example to understand how these two concepts work:
A random sample of Americans were asked whether they preferred cake or ice cream for dessert. The results showed that 60% preferred ice cream over cake, and 40% preferred cake over ice cream. This question had a margin of error of +/- 3% at a 95% confidence level. What this means is that you can be 95% confident that the percentage of all Americans who prefer ice cream would fall between 57% and 63% (60 plus or minus 3). Another way to put it would be as follows: if you conducted 100 surveys of the entire population, at least 95 times you would find that the percentage who preferred cake ranged from 37% to 43%.
So what the heck does that mean?
Well, the first part basically tells you how close the results from your selected sample are compared to what you'd find if you surveyed the entire population. The expression of “plus or minus x%” tells you that the percentages of given responses might be a bit higher or lower “in reality” (i.e., if you surveyed absolutely everyone).
Generalizability to the larger population is also described by an associated measure called a confidence level (CL). This term describes how confident you can be that your results are not due to chance alone. A confidence level is normally set at either 90%, 95% or 99% (95% has become standard).
Let’s use an example to understand how these two concepts work:
A random sample of Americans were asked whether they preferred cake or ice cream for dessert. The results showed that 60% preferred ice cream over cake, and 40% preferred cake over ice cream. This question had a margin of error of +/- 3% at a 95% confidence level. What this means is that you can be 95% confident that the percentage of all Americans who prefer ice cream would fall between 57% and 63% (60 plus or minus 3). Another way to put it would be as follows: if you conducted 100 surveys of the entire population, at least 95 times you would find that the percentage who preferred cake ranged from 37% to 43%.
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