# How to specify window length in formula language?

Some formulas require you to define a window length.

Let’s take as example the formula for a rolling mean:

Mean(series, window)

For the window length you should input the number of observations over which the function will be calculated. The number of observations is based on the frequency of the document.

Instead of typing a number, you may also use other formulas to get the number of observations in a specified period, such as YearsLength(1), QuartersLength(4) or MonthsLength(12).

For example, you can calculate the one year rolling mean of a series, by using the following formula:

Mean(series, YearsLength(1))

The advantage of this is that it is not necessary to concentrate on the frequency of the document in order to determine the number of observations.