yeah, no surprises there...... thank you Ian James for stating the blatantly obvious.
If you take a sample of 11 out of a huge number (no. of houses, not only ones being sold) and find a measure of the central price (median in this case) and then compare it to a sample of 7, you will often find different central prices.
However if you took a sample of 10,000 and compared to the median of a sample of 9996 it would usually be quite similar....... a monkey can understand this....
So anyone without their head in the sand would realise you need bigger data sets than 11.......
Anyone else needing to relearn high school maths can do so by reading the wiki page for the law of large numbers.
