# Length of recall period during SMART survey

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### Rogers Wanyama

Emergency Nutrition Specialist

Normal user

11 Feb 2013, 11:19

### Mark Myatt

Frequent user

11 Feb 2013, 13:32

```
n(pdar) = u / (precision / 1.96)^2
```

where "u" is the expected rate and precision is the desired half width of the 95% confidence interval.
An example ... we expect a rate of 2 deaths / 10,000 / day (i.e. 0.0002) and want to estimate this with a precision of plus of minus 1 death / 10,000 / day. This gives:
```
n(pdar) = u / (precision / 1.96)^2
n(pdar) = 0.0002 / (0.0001 / 1.96)^2
n(pdar) = 76,832
```

If you use a cluster sampled design then you can expect a rather high design effect (e.g. because mortality due to infection will be clustered). The expected design effect (DEFF) is usually set at 2.0. The sample sizes calculated above should be multiplied by the expected design effect. Continuing the example we have:
```
n(pdar) = DEFF * [u / (precision / 1.96)^2]
n(pdar) = 2.0 * [0.0002 / (0.0001 / 1.96)^2]
n(pdar) = 153,664
```

This is a large number. There are two ways to get at this sample size : We can increase the number of people followed up or we can increase the length of the recall period. It is not usually sensible to increase the length of the follow-up period above about three months. Continuing the example:
```
n(persons followed for 90 days) = 153664 / 90 = 1708
```

This can be translated into households to be sampled. If (e.g.) the average household size is 4.5 persons then:
```
n(HH) = 1708 / 4.5 = 380
```

This is readily achievable so you may want to reduce the follow-up period and sample more households.
BEWARE : The sample for a SMART nutrition survey and a mortality survey will be different. If you use the same sample you risk underestimating mortality by excluding households in which all children have died. This is a survivor bias.
Another consideration is memorability of the start of the recall period. If you can use (e.g.) "since the start of the new year" or "since Christmas" then you are more likely to have deaths reported within the recall period and reduce recall bias.
You can find a review of some of these ideas in this Field Exchange article.
I hope this helps.
### Anonymous 757

Nutritionist

Normal user

11 Feb 2013, 15:33