JMP Weighting Cases Screenshot Tutorial

Cases are weighted when the sample sizes for a particular variable (e.g. gender) are disporportionate to the population.  For instance, if we have a sample that includes 47% boys and 52% girls but the population from which that sample was drawn contains 40% boys and 60% girls , we might opt to weight our data to adust the distortion in the sample.  We would begin by calculating the weighting factor.  This is done by determining the proportion of the respective group in the population divided  by the proportion of that group in the sample.   

Weighting factor = % of population / % of sample  

In our above example:

• Weighting factor for girls = 0.60 / 0.52
• Weighting factor for girls = 1.15
• Weighting Factor for boys = 0.40 / 0.47
• Weighting Factor for boys = 0.85 

Once we have determined the Weighting factor, a new variable can be created.  But first, we need to recode our character data to numeric.

To recode a column (variable).  Click on the Column hot spot (red down arrow).  Click Recode.

Type in the New Values. 

Click on the down arrow for "In Place"

In this case, we simply changed the codes within the same column ("in place").  Click on the Column and then Click Column Info.

Change the Column Data Type to "Numeric".

Click on a blank Column to create the new Weighting Variable.  The above window will pop-up.

Change the column title to "Weighting Cases".  Click Column Properties -- Formula.

The formula window will pop-up.  Click on the Conditional Function above.

The formula will appear as Match [expression][value -- then cause] as you see above.

Click on the symbol type pad -- up arrow symbol.  It will change the formula to the above.

Click on the symbol type pad -- Click the up arrow symbol again.  It will change the formula to the above.

Click the Table Column "Gender" and place it in the Expression part of the formula.  Then Click on the 'value' and type 0 (for the boys).

Type the weighting factor for boys (0.85)  then click the next value and type 1.0 for girls.  Finally, click the "then clause" and add the weighting factor for girls (1.15).

Click Apply and then O.K.

A new variable (column) "Weighting Cases" has been created. Note the weights.  Be sure that they correspond with the appropriate gender.

To use the weighting Column, click Analyze -- Distrubution.

For additional questions or comments contact Tina Ughrin