|
WHEN quotas for telephone interviewing projects are set and clients are the source of the sample, one of the most common questions we get is "How many numbers do you need?" While suppliers would like to have as many numbers as possible, here is a simple guideline for you to follow to determine the minimum amount necessary: Multiply the number of interviews to be completed by 5 and divide by the net incidence of qualification for the study. That's it! Pretty simple, huh? And the best part is, it works! Let's try an example.
Suppose you wish to complete 500 customer interviews and the incidence of qualification is estimated to be 60%. You would need approximately 4,200 telephone numbers as illustrated in the formula below.
(500 x 5) / .60 = 4,167
Please note that, like most formulas, there are a few exceptions to the rule. If you happen to have a research study that deals, for example, with a particularly sensitive topic, if your survey is particularly lengthy (more than 15 minutes) or if you wish to complete a large number of interviews in a short amount of time (e.g. overnight), you will likely require considerably more numbers than the formula would dictate.
The age of your sample must be considered as well, as it is not accounted for in this formula. Remember, if your sample list is outdated, it is more likely to include non-working or unproductive numbers, resulting in a need for an even greater ratio of phone numbers to completes than the formula would otherwise suggest.
|
