Parameter is a quantity of interest from a population. Therefore, it is often unknown (unless we can enumerate the entire population) and needs to be estimated by drawing a sample from the population.
To examine the mean change of blood pressure (BP) after taking a anti-hypertensive drug in hypertensive patients, we may randomly recruit a number of patients and measure their BP before and after the administration of the drug. The true mean BP change is the parameter of interest. It is unknown and needs to be estimated.
A researcher wants to determine the proportion of Hong Kong school children with habitual snoring. A sample of 3047 school children was contacted by phone. Of which 45 (1.5%) children were found with habitual snoring. The corresponding standard error was 0.0022.
The proportion of Hong Kong school children with habitual snoring is the parameter of interest. Again, it is unknown and needs to be estimated.