Pattern 65: Equal opportunity [Abstract]
 AKA:

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How do you make users feel they control the tasks they are trying to perform?

Therefore

Try to design interactions that users may perform on your site to give them equal opportunity in the selection of the order or entry of parameters.

Equal opportunity is closely related to NO MODES (40).

Discussion - forces - known uses

Equal opportunity means that output can become input and vice versa; e.g. aperture or shutter priority cameras.
Thimbleby (1990) gives the example of calculators of the sorts illustrated above. Standard calculators are not equal opportunity because the result and the operands are not interchangeable. For example, I can put in ‘13 * 4 =’ and will (with luck and a good battery) get ‘52’; but I cannot put in ‘? * 4 = 52’ and expect the unknown to be computed as ‘13’. Incidentally, these devices also have poor feedback in the interface. If you want to check that 0 * 0 = 0, you may equally well believe that the device is not working as that the algebra is vindicated. The improved design is based on equal opportunity; set the symbols for the two button displays and put in any two numbers, and the third will display a consistent result. We could further improve the design by not using 0 as the default display.

Another, better-known example of equal opportunity is Query By Example (QBE), a widely used query method for relational and pseudo-relational databases. QBE offers the user a blank form such as the one below. The user may then type fixed values for Dept and Salary and QBE will return all records which match the pattern. This is not dissimilar in principle from what occurs with Prolog queries which are resolved by pattern matching too. The equal opportunity arises because the user may choose any value to fix and is not constrained to a fixed, pre-designed dialogue, such as:
Enter Dept: Sales
Enter Salary: >10,000
<list of matching records>

Figure 65.1 Query By Example in database enquiry systems.
Name Dept Salary
? Sales >10,000
     

Cut and paste facilities usually give equal opportunity, or at least should do.

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