Outliers

Outliers are values that are far from the central tendency. Outliers might be caused by errors in collecting or processing the data, or they might be correct but unusual measurements. It is always a good idea to check for outliers, and sometimes it is useful and appropriate to discard them.

In the list of pregnancy lengths for live births, the ten lowest values are {0, 4, 9, 13, 17, 17, 18, 19, 20, 21}. Values below 20 weeks are certainly errors, and values higher than 30 weeks are probably legitimate. But values in between are hard to interpret.

On the other end, the highest values are:

weeks  count
43     148
44      46
45      10
46       1
47       1
48       7
50       2

Again, some values are almost certainly errors, but it is hard to know for sure. One option is to trim the data by discarding some fraction of the highest and lowest values (see http://wikipedia.org/wiki/Truncated_mean).