Sampling

Why sample?

• To make an inference about a population

• Studying entire pop is impractical or impossible

Example of sampling

• Estimate the proportion of adults, ages 18-65, in Port Elizabeth that have type 2 diabetes

• Select a sample from which to estimate the proportion

• Population:  adults aged 18-65 living in Port Elizabeth

• Inference:  proportion with type 2 diabetes

Probability sampling 

• Each individual has known (non-zero) probability of selection

• Precision of estimates can be quantified

Non-probability sampling

• Cheaper, more convenient

• Quality of estimates cannot be assessed

• May not be representative of population

How many hours on internet?

This one of the question being asked to me on an online survey. Literally, you can ask yourself, how many hours you spend your time to access internet in a week?
I answer that question by calculating hour spending on internet in a day multiply day spending on internet in a week. Guess what, I spend at least 5 hours and 7 days. So, it will be 35 hours a week.
That is minimum number. Sometimes I spend more than 5 hours a day just to access internet.
Ask yourself that question and begin to count.