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A common misconception about ergonomic tests is that it always requires a lot of participants to get valuable feedback about the design. In everyday practice however asking seven to eleven persons can already reveal the biggest problems in a design. Here is the proof why.

The reason why we need only so few persons and already get valuable data is because we are looking for the most common problems in an early stage in productdesign. What we want is to tackle the biggest issues with our new product design at an early stage possible. To do this we can already suffice with a small amount of participants. The results from this test will not help you to tackle all problems but will help you to find the most common problems in this early stage.

An analogy, say in a fishpool you see mainly orange-white spotted koi-fish, despite the fact that there are full-orange and full-white koi fish existing in nature as well. Without overseeing the entire fishpool, it would be reasonable to assume that other fish in this pool are similar orange-white spotted, not? This analogy works as well in productdesign: the first seven to eleven people we ask to give feedback during a test are most valuable (quite likely the last three people interviewed will merely repeat and reconfirm what has been said already).

The number of participants required for a usability test to get a certain percentage of  usability problems found in a product can be studied using a binomal model as well. If you are interested in these statistics I advise you to read an article by H. Kanis How many participants - Departement of Industrial Design, Delft, Netherlands. The article questionned the initial rule-of-thumb of using only five people for trials, leading to the conclusion that the rule-of-thumb of five might have been too optimistic. Based on practical verification it can be concluded that using seven to eleven people does cover 80% of all usability problems.

The big benefit of verifying our design like this, is that it only requires a few days to a week to test and gives a lot of valuable data. Data to redesign or data to go for a bigger test afterall. This way of testing can be applied to different kind of products: interfaces, software, new product concepts, products already on the market, etc.

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