I have given you 3 explainations now! haha I'll try to be clearer I guess.
Lets assume that the population of 21 year olds is 100. These 100 people take the WAIS. Some do well, some do poor, some do ok, some do great. Lets say the test was out of 100 points and the mean was a 60. We now take that mean score and say whomever got closest to that (in a real population many people would get exactly that) has an IQ of 100. Roughly 50 people will have a score higher than 60, and roughly 50 people will have a score below 60. Now we further divide those people up using standard deviations. In most tests, 15 is the standard deviation. This means 68% (68 people) out of our population of one hundred 21 year olds will have an IQ between 85 and 115. ALWAYS, this is the nature of the bell curve. 95% (95 people) will have an IQ between 70 and 130. ALWAYS, again, this is the nature of the curve. We can go on..
Now we wait 40 years and have this same population, now 61 years old, take the test again. Some do well, some do poor, some do ok, some do great. Again, the test was out of 100. This time, the mean was 68 (or anything, it doesnt matter). We again take that mean score and whoever got it, has an IQ of 100. Still, roughly 50 people have scores above and 50 have scores below. Again, we further divide them up. 68 people have IQs between 85 and 115, 95 people have scores between 70 and 130... etc..
Notice that the same amount of people achieve each IQ level regardless of how each individual scores on the test. The mean will ALWAYS be 100, and the same amount of people will ALWAYS have scores in each standard deviation. Thus, scores DO NOT tend to 100.
Please tell me this made sense... I dont know if I can break it down any further for you //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
[edit for some spelling...]
