# Weighting prepare and calculate

See Weighting respondents for an overview of rim/target weighting.

This section describes how to calculate the weights for respondent weighting using targets.

## Prepare and check data

Before the calculations can begin you need to prepare the data with [Calculate] [Prepare and check data]. This checks the data for all the targets and sets all the weights to 1.0.

If any targets have no usable data you should set these targets to "Ignore". You may have to redistribute the target to other responses so that the total remains the same (usually 100%).

If the data has been weighted earlier you can choose to start with those weights rather than starting every record with a weight of 1.0. If any records have been added since the weights were calculated you will need to manually set their weights to 1.0. This option could be used for "two stage" weighting.

## Calculate weights

[Calculate] [Calculate weights] will start a number of iterations (passes) over the data adjusting the individual weights to try to reach the targets.

This process will continue until all the targets have been reached or no further progress can made.

If the number of iterations is excessive then there is probably a problem with the targets or the data. You can stop the iterations at any time and inspect the summary (top of window) or the individual data records (bottom of window) to see where the problem lies.

## Precision

In normal market research the weighted totals do not
need to be **exactly** the same as the targets. A
total that is very close is good enough. The
default precision is 8 significant digits, meaning that the first 8 digits of the weighted total will be the
same the target. The program will cease iterations
when every total is this close to its target.

If your weighted totals are very large you may want increase the precision. You can do this after calculations have been done and then use [Calculate] without [Prepare and check data]. You only need to [Prepare and check data] once unless you have changed the targets, the weight question, or the data file.

The higher the precision the more iterations will be needed and the longer it will take to calculate the weights.

The individual weights are stored in the float question and some precision may be lost if insufficient data locations have been allocated to the question. To see the values that will actually be stored use [Calculate] [Show stored weights].

## Saving

If you close the Weight data window without saving the data you will be prompted to do so.

**Important:** The newly calculated
weights are not stored in the data until you save the
data file. Before you do this you should check and save
any reports that you might need.

Once the weights have been calculated you should check the summary report at the top of the window