For ungrounded data

the population mean:

the sum of the population values divided by the number of population values.

μ=∑X/N

• μ is the population mean
• N is the number of observations in the population
• X is a particular value
• ∑ indicates the operation of adding

The sample mean 

the sum of the sample values divided by the number of sample

Parameter:

a measurable characteristics of a population.

Example:The Kiers family owns four vehicles.
Following is the current mileage for each:
56,000; 23,000; 42,000; and 73,000.
Find the mean mileage .
The mean is (56,000 + 23,000 + 42,000 + 73,000)/4 = 48,500

Statistic:

a  measurable characteristic of a sample.

Example:A sample of five executives received the following amounts of bonus last year:
$14,000, $15,000, $17,000, $16,000, and $15,000.
Find the mean bonus for these five executives.
Since these values represent a sample size of 5,
the sample mean is (14,000 + 15,000 +17,000 + 16,000 +15,000)/5 = $15,400.