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What Is a Decision Table?

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  • Written By: Mark Wollacott
  • Edited By: Amanda L. Wardle
  • Last Modified Date: 28 September 2014
  • Copyright Protected:
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    Conjecture Corporation
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A decision table offers a succinct way of organizing data and linking conditions and actions together. It is sometimes used as an alternative to flowcharts and switch-case statements. Decision tables may be simple or complex, and are often used to address complicated programming and business decisions by presenting the conditions and actions in a simple and elegant form.

The decision table is usually divided into at least four quadrants. The most basic example has two rows and two columns. The top left quadrant houses the conditions, while the top right is used to outline the condition rules and alternatives. The bottom left quadrant, meanwhile, contains the actions, and the bottom right the action rules. Decision tables vary quite a bit beside these four basic quadrants.

The conditions row is used to assess the situation. For example, a table may be used to diagnose a problem with a television. The first column in the top left section of the table may simply say ‘conditions,’ while the second column is sub-divided into different rows with one condition per row. Television related conditions could include: no signal, no picture, dim backlight, and no sound.

The action section, on the other hand, describes possible solutions to the problems outlined in the conditions. The first column will simply say ‘actions,’ while the second is sub-divided into rows containing actions the person can take. For the television this might include: checking the aerial cable, checking the plug, and adjusting the volume control.

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A common addition to the table is the rules checklist. A simple checklist would contain Y for yes and N for no. There is one rule column for each of possible combinations of conditions. For example, if there are four conditions for the television, then there are 16 possible combinations of Y and N. If there are only three conditions, there will be eight possible combinations.

The person using the decision table will tick, circle, or mark the Ys from the conditions list to find which of the combinations is correct. Depending on which conditions are met, there will be Xs marked next to the actions that need to be taken. This means the decision table directly simplifies the decision-making process.

In order to develop a well balanced decision table, the creator needs to decide the most relevant conditions to list. There is no limit to the number of conditions that can be listed, but each additional one adds to the number of rule combinations used to decide what actions need to be taken. Rules should also be limited to the most relevant when compared to the listed conditions.

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