Population & Sampling
Population is the group you want to find out about (eg all girls in school; all cars in UK)
A Census is information about every member of the population.
A sample of data is collected from a part of the population in order to make conclusions about the whole population.
- Advantages: practical, cheaper, quicker than doing a census.
- Disadvantages: don’t have information about every member of the population. May not be representative of the population.
The Sampling Units are the peple or items taht are to be sampled.
The Sampling Frame is a list of the people or items that are to be sampled.
Simple random sampling – every member of the population has an equal chance of being selected. Advantage unbiased. Use a random number table or number the population and pick random numbers fromthe ones.
· Systematic sampling – Chose a starting point on yur sampling frame and select at regualr intervals. (divide population size by sample size eg 2000 ÷ 50 = 40. Randomly choose a start number between 1 & 40 say 3.) Should produce an unbiased sample unless there is some sort of pattern in the data.
· Stratified Sampling – split population into groups (strata) & choose a number from each group proportional to the size of the group in the population. Useful if you have easy to define categories eg age, gender. Sample size= total year group/ total population x sample size
· Cluster sampling – Used when a population splits naturally inoto clusters or groups. Samples are taken from each cluster.
· Quota sampling – often used in market research. Divide population into groups based on age, gender etc. Interview a certain number of people from each group. Disadvantage: Can easily be biased as sample chosen depends on interviewer.
Survay : A collection of data from a given population and used to analyse a particular issue.
Pilot Survay: Used on a small sample to test design and method.
Questionaire's: Set of questions used to collect data.
Open question: Question with suggested answers.
Closed question: Queation with a set if answers to choose from.
Data Collection 2
Survey's- When doing a survey you may attain information by: -questionaire, interview, observation,experiments and data logging
Questionaire's - Questions should be: -Short and to the point - Understandable -Not leading -Adress single issues - Including unit -Not embarr***ing
Interview's- Cna be face to face , By telephone and by post or email
Investigations/ Experiments- Counting cars that *** your house. Could be done before and after a speed restriction is applied.
Control groups- Often used to check the effictiveness of a medicine. One group takes the drug, the contol group doesn'tand the results are compared.
Capture- Recapture- To estimate a pouation size. Formula N= Mn/m
Measures of spread