One advertisement recently for a fertility clinic proclaims that 70% of successful clients became pregnant within two months of enlisting their services. What is not clear is just how many of their clients were successful at all - and how many of those successes were due to the skills of clinic staff and how much was up to the vagaries of nature. Presumably we were meant to hear this statement simply as: "70% of clients became pregnant within two months" with the keyword successful removed. If a client later says they were misled, the clinic can honestly say it's all there in black and white. The fact that the advertisers see fit to feed us this particular figure is almost an iron clad guarantee that the percentage representing those who became pregnant at all in some reasonable time frame is somewhat less than 70% - else they would quote that figure instead.
In another advertisement, this time for a breakfast cereal which is supposedly good for one's figure, we are told that a number of women are 'challenged' to try the cereal for two weeks and see how they feel. After this time the women are presumably interviewed for we are told that eight out of ten women would be willing to take the challenge again. Given that the cereal is unlikely to do anyone any serious damage it is hardly an impressive endorsement that people are willing to say that they could cope with this challenge once more.
Perhaps the silliest example that comes to mind was an advertisement for a cosmetic cream which presented the ludicrous idea that the product on offer was 50% more effective at making the user more beautiful than the products offered by their competitors. The question that comes to mind here is: how does one create a precise mathematical formula to describe the very the effects of a cosmetic on such a subjective thing as beauty? While it is true that certain aspects of beauty can be described mathematically - such as the ideal ratio of waist to hip width - very often opinions on beauty have as much to do with culture and fashion as anything else. Waist-hip ratio gives a clue to a woman's ability to bear children and thus it may be used to measure the beauty of a woman in regard to this one particular trait, but equally we cannot apply mathematics to determine the ideal BMI for a 'beautiful' woman since a quick review of cultures throughout history tells us that there is little agreement in this area.
In recent years a rather insidious example of abused percentages comes from the food industry. A foodstuff may be described as being 98% 'fat-free'. Great! There is fat in only 2% of this product! We possibly wonder what proportion of the 2% is fat - of course the answer is: 100%. The buyer is comforted by the term 'fat-free' despite the fact that the '70% fat-free' chocolate they are eating is clearly a very fatty item. Another related point comes from observing the descriptions of fat content on the packets of chocolate from the one manufacturer. A single 'serve' of chocolate contains 8-10% of the recommended daily intake of fat. However, a 'serve' is only 25g or five small squares on plain chocolate or only two or three squares for fancier varieties. Of course any self-respective chocoholic knows that two or three squares is a laughably inadequate serving and thus it is very easy to obtain a very significant portion of ones recommended daily ration of fat during a single movie.
Whenever confronted with percentages describing the qualities of a product, these figures should be treated with suspicion. Advertisers will always do their best to display a product in its best light - that's their job afterall. If the numbers seem too good to be true they probably are. Take a moment to re-read main assertions to be sure you haven't ignored a critical modifier or given too much attention to a comforting term like 'fat-free'. Also pay attention to the fine print - assuming of course you can find the find print.* Think about whether the quality being described can objectively accept a numerical score.
*It is remarkable how often wonderful deals are associated with asterisks for which the associated footnote has taken advanced camouflage training.