Determining the length of the recall period
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How should you decide the length of the recall period?
- The shorter the recall period, the more up-to-date is your mortality rate. The mortality rate measured in a survey is the average mortality rate during the entire recall period. As a result, if you want to know recent mortality, the average mortality rate for the past 3 years is probably not of much use. In most acute humanitarian emergencies, a recent morality estimate is much more useful than an older one.
- Because the denominator of mortality rates is person-time, the longer the recall period, the more person-time units are in the denominator of the mortality rate. This means that the longer the recall period, the bigger the sample size (in person-time units) Therefore, the longer the recall period, the more precise will be your estimate of the mortality rate if the same number of households are selected for the survey sample.
- The recall period must be carefully defined to include or exclude specific seasons. If you include a deadly season of the year, such as the rainy season when mosquito breeding takes place and malaria transmission is greatest, you will end up with a higher mortality rate. If you wish to compare this rate to prior estimated rates which did not include this season, you may falsely conclude that the risk of death has increased for some other reason when the apparent increase is only due to seasonal variation.
But the bottom line is: What is your mortality question? If the question is "What is the crude mortality rate now?" then you should define a short recall period to estimate mortality in the more recent past. If the question is "What has the crude mortality rate been for the past 12 months?" then your recall period should be much longer.
Refugee camp A has never had very good prospective mortality surveillance, but the impression is that mortality has been stable for a long time. I want to know what the crude mortality rate has been for about the past year. What should the length of the recall period be?