by: Dick Stroud

My favourite Web usability guru, Jakob Nielsen, has published a fascinating article about the impact of ageing on the way people use the Web. Do make sure you read the full article.

This is a summary of the article’s conclusions.

Between the ages of 25 and 60, the time users need to complete web site tasks increases by 0.8% per year.

I suspect this is a sentence that you are going to read time and time again.

This means a 40-year-old user will take 8% longer than a 30-year-old to accomplish the same task. And a 50-year-old user will require an additional 8% more time. Note this increase is linear, not exponential.

This degradation is mostly because as people age they spend more time per page, but also because of navigation difficulties.

This finding is statistically significant at the 5% level, given the 61 users in his study.

Another of Nielsen’s conclusions is that individual differences swamp age-related difference in the 25- to 60-year-old group. Users are extraordinarily variable in their use of web sites.

He has something he calls a 5-5-5 rule for the way users complete web site tasks:
• the slowest 5% of users are
• about 5 times as slow
• as the fastest 5% of users.
Thus the slowest users need 400% more time to perform the same tasks. The 0.8% difference caused by each year of aging pales in comparison. A fast 50-year-old will beat a slow 30-year-old every day — by several hundred percent.

Because of cognitive ageing older users need more time to understand pages, scan the text, and extract the information. A smaller — but still substantial — problem is that people have more trouble navigating websites as they age.

The human ageing process causes erosion of cognitive resources, loss of visual acuity, degraded reaction times, and reduced dexterity. People need more time for the same mental operations; they have less memory capacity and take longer to process the same perceptual input.

All of these elements of human performance impact the speed with which users can get something done on a web site.

Because the Web is relatively new, a 50-year-old might have started using it at age 40, whereas a 30-year-old might have started at age 20. In contrast, by 2050, a 50-year-old will have used the Web since age 5, and thus benefit from 45 years of experience. Nielsen believes that this added web experience might eventually allow older users to catch up and somewhat reduce the 0.8% gap – his guess is that the age penalty will drop to around 0.5%/year. I think he would agree that this is a "finger in the air" guess.

Nielsen has already published research that shows that the 75+ are 74% slower using websites than mainstream users. A typical senior at 75 is 40 years older than a typical mainstream user at 35, so 0.8% per year should correspond to only a 32% slow-down for seniors.

He believes the differences are explained by the fact that aging starts early, but accelerates drastically around 60 years of age, and especially after 70 years. Curves of cognitive, perceptual, and motor-skill decline have a hockey-stick shape not a straight line.

So his 0.8%/year slow-down is valid only for the mainstream period of 25–60 years of age. For older users, performance declines faster.

Nielsen’s bottom line is that you need separate guidelines for seniors and truly young users; you don't need different usability guidelines for your 50-year-old vs. 30-year-old customers. Finally he advises that when doing user testing, make sure to include test participants across the entire age range you're targeting and don't believe everything your 25-year old Web designers tell you about "what's easy" — especially if your target audience is 50-year-old corporate managers!

It would be good use of your time to read the original article.

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