# 15.7 Output group-level stats from dispersion (or variability) weighting

In PRIMER 8, you can now output more detailed statistical information from either dispersion-weighting or variability weighting to a worksheet. You can see this additional option in the comparison of the 'Dispersion Weighting' dialog in PRIMER 8, compared to that in PRIMER 7 (Fig. 15.7).

[![10c._DW_comparison_P7_P8.png](https://learninghub.primer-e.com/uploads/images/gallery/2025-07/scaled-1680-/10c-dw-comparison-p7-p8.png)](https://learninghub.primer-e.com/uploads/images/gallery/2025-07/10c-dw-comparison-p7-p8.png)

*Fig. 15.7 Comparison of the **Pre-treatment** > **Dispersion Weighting...** dialog window in PRIMER 7 (at left) vs PRIMER 8 (at right).*

In essence, whenever we calculate the average ***index of dispersion*** ($\bar{D}$) for the dispersion weighting pre-treatment, it is drawn from individual indices of dispersion (variance-to-mean ratios, $D_i$) for each of several ($i = 1, \ldots, g$) groups (identified by a factor). It would be helpful, in some situations, to be able to see those original individual indices of dispersion across all of the groups for each variable. The extra tickbox ('$\checkmark$Output group-level worksheet') available in the PRIMER 8 dispersion weighting dialog allows you to produce and examine this underlying statistical information in a worksheet directly.

For example, we can re-visit the data we saw in [section 10.4](https://learninghub.primer-e.com/link/1049) above, located in the file '<ins>NE_NZ_fish_counts.pri</ins>', found in the '<ins>Example_P8</ins>' > '<ins>NE_NZ_fish</ins>' folder. We can begin by following steps 1, 2 and 3 from [section 10.4](https://learninghub.primer-e.com/link/1049) on these data; i.e., get the data into PRIMER 8, then:
- Obtain a subset of the data based on the factor of '<ins>Year</ins>', from 2010-2015 inclusive (**Select** > **Samples...**), with the resulting subset being called '<ins>Data1</ins>'.
- Create a combined factor consisting of all combinations of the factors: '<ins>Loc</ins>', '<ins>Hab</ins>' and '<ins>Year</ins>' (**Edit** > **Factors...** > **Combine...**)

Next, we can apply the dispersion weighting pre-treatment on the basis of this combined factor by clicking **Pre-treatment** > **Dispersion Weighting...** and in the resulting dialog, choose Factor: <ins>Loc-Hab-Year</ins>. This time, however, we shall also choose to get ($\checkmark$Stats to worksheet) & ($\checkmark$Output group-level worksheet), then click '**OK**'.

This will produce the following three worksheets (assuming '<ins>Data1</ins>' contains the subset-selected data):
- '<ins>Data2</ins>' contains the dispersion-weighted data.
- '<ins>Data3</ins>' contains the statistics associated with the ***average*** index of dispersion ($\bar{D}$) for each variable.
- '<ins>Data4</ins>' contains the ***individual*** indices of dispersion ($D_i$) for each group (columns) and for each variable (rows).

These three data sheets are shown below for the present example.

[![10d._DW_data_[i].png](https://learninghub.primer-e.com/uploads/images/gallery/2025-12/scaled-1680-/10d-dw-data-i.png)](https://learninghub.primer-e.com/uploads/images/gallery/2025-12/10d-dw-data-i.png)

[![10e._D-bar_values_fish_[i].png](https://learninghub.primer-e.com/uploads/images/gallery/2025-12/scaled-1680-/10e-d-bar-values-fish-i.png)](https://learninghub.primer-e.com/uploads/images/gallery/2025-12/10e-d-bar-values-fish-i.png)

[![10f._D_i_values_fish_[i].png](https://learninghub.primer-e.com/uploads/images/gallery/2025-12/scaled-1680-/10f-d-i-values-fish-i.png)](https://learninghub.primer-e.com/uploads/images/gallery/2025-12/10f-d-i-values-fish-i.png)

Note in the above sheet ('<ins>Data4</ins>', containing individual indices, $D_i$) that there are many missing values. This is simply a consequence of there being zero fish of that species in that particular group of samples; hence, the mean and the variance will both be equal to zero, so no value of $D_i$ can be calculated for that particular cell (or group). Average $\bar{D}$ values for each species are naturally calculated only from those cells (groups of samples) where at least some individuals of that species were recorded (i.e., the non-missing entries).

Finally, note that this '$\checkmark$Output group-level worksheet' option is also available for **Pre-treatment** > **Variability Weighting...**, *viz.:*

[![10g._Variability_Weighting_dialog_too.png](https://learninghub.primer-e.com/uploads/images/gallery/2025-07/scaled-1680-/10g-variability-weighting-dialog-too.png)](https://learninghub.primer-e.com/uploads/images/gallery/2025-07/10g-variability-weighting-dialog-too.png)