Houston Skyline
Suppose you randomly sampled 10 people from the population of women in Houston, Texas between the ages of 21 and 35 years and computed the mean height of your sample. You would not expect your sample mean to be equal to the mean of all women in Houston.
Source
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"All sizes | Houston Skyline | Flickr - Photo Sharing!."
http://www.flickr.com/photos/mrbill/4803461/sizes/o/in/photostream/
Flickr
CC BY.