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# Difference (Gap) Analysis

Updated: Oct 2

The goal of this article is to help you learn how to compute the difference between two measure fields in reference to a dimension in Tableau. A good business question would be, computing the difference (gap) between demand and supply over different months of year.

Using **Traffic Incidences** data sets, we’ll recreate the chart below showing the existing gap between the number **Injured** and **Deaths **recorded for different **months**.

**Note:** This article is for learning purpose, but you’ll find applicable scenarios to use this technique in real life.

**Create a side–by–side bar chart**

Once connected to the data set in the link above;

Change the String dimension field

**Date YMD**data type to**Date**.Drag now Date dimension field

**Date YMD**to the columns shelf, choose level of details as**month**.Drag both measure fields

**Deaths**and**Injured**to the rows shelf.Change the chart to

**side-by-side bars**by selecting**side-by-side bars**under**Show Me**tab.

Resulting view;

Create calculated field – **Gap **- to compute the difference between the number **Injured** and **Deaths**.

Create a calculated field – **Gap condition** – to inform how the gap will be interpreted.

Create a calculated field – **Absolute gap** – to eliminate negative values in the gap.

Create a calculated field – **Main gap **– to determine where the gap will begin from.

**Note:** The implication to this calculation is, if SUM(Injured) is greater than SUM(Deaths) then let the gap begin at SUM(Deaths) else let the gap begin at SUM(Injured).

**Add calculated field -***Main gap* – to the view

*Main gap*– to the view

Drag the measure field

**Main gap**to the rows shelf.Make the chart

**dual**and**synchronize**the axis.On the legend, sort the fields by dragging field

**Main gap**to the center.

Change the second chart - **Main gap** – to a **Gantt Bar** chart.

**Fill the gap**

To fill the gap between the two measure field.

While still at the second chart **Gantt Bar** drag calculated field - **Absolute gap** – to the **size** shelf.

Add visual cue to differentiate the nature of gap by dragging the calculated field – **Gap condition** - to the **color** shelf of the second chart and customize colors appropriately.

**Label the gaps**

While at the second chart, drag calculated field – **Absolute gap** – to the **label** shelf, align the labels and make any other customization you wish.

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