Marketing Research Blog

As we are getting so close to the presidential election, it is time to mention margin-of-error once again. Just yesterday, I heard one of the radio personalities on Chicago 890 AM—WLS comment that many people didn’t understand the concept of margin-of-error. And that’s true.

Then he proceeded to explain what it meant. In doing so, though, he got it just half right. The radio personality gave a hypothetical example that 50% of voters were likely to vote for one of the candidates–with a margin-or-error of plus or minus 5 percentage points. He described that his statement meant that the “real percentage” was in the range of 45-55%–which is correct. But he failed to indicate how likely it was that is the “true score” fell in the 45% to 55% range. If nothing else was stated, the confidence limit (the degree of certainty the “true value” was in the range was 95 %.).  Furthermore, because of the size of the range + or - 5%, it tells us that the sample size was considerably less than 1100.  It is much more frequent that we see margins-of-error of + or - 3%, as the radio personality’s colleague had indicated. When the statement is made that 50% preferred one candidate–with a margin or error of 3% (+ or - ) 3%, this will always mean that one can be confident that 95% of the time when such a survey is undertaken the actual score will be within + or - 3% of the observed score (which in this case would have been 50%). This would also imply that the research had used a sample of approximately 1100 individuals–hopefully individual that were representative of the population under study–such as “likely voters.”

This entry was posted on Thursday, October 9th, 2008 at 11:44 am and is filed under Polling. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.