1. Collecting Data

2. Types of Data Quantitative data Qualitative data Primary data
This data is numerical. This means the data uses numbers.
e.g. the price of an object Age
Qualitative data
This data isn’t numerical. This means it doesn’t use numbers.
e.g. colours
Primary data
This data is collected by the person who will use the information.
e.g. results of a questionnaire taking measurements
Secondary data
This data is not collected by the person who will use the information.
e.g. obtained from published statistics/databases data found in textbooks or on the internet

3. Sampling Population Sampling frame Census Bias Sample
The complete set of people/objects that the information is collected about.
Sampling frame
A list of all the items of the population.
Census
When data is collected from every person or about every object.
Bias
Anything that distorts data so it is not a fair representation. This makes results unreliable.
Sample
Information is usually taken from a small part of the population. This is a sample of the population. It is easier and cheaper to take a sample rather than a census.

4. Sampling Techniques Random sample Systematic sample
Every member of the population has an equal chance of being selected.
Random samples can be found using:
Writing numbers on paper and choosing a number at random
Random number tables
Random number generator on scientific calculators
Systematic sample
Every member of the sample is chosen at regular intervals from a list.
To find a start point, a random number needs to be chosen to remove bias, however the order in the list might have been deliberately made instead of having no order.

5. Sampling Techniques Convenience sampling Stratified sampling
This is when someone is conducting a questionnaire and asks a certain number of people. This can be biased if the person chooses who to ask or if people don’t participate.
Stratified sampling
If a population is divided into groups, the groups must be fairly represented in the sample. The number chosen from each group is proportional to the group size.
Quota sampling
This sampling method is often used in market research as it is very cheap but unreliable. The interviewer asks certain people that must be of a certain type. This could be age, sex or social class.
Cluster sampling
The population is divided into smaller groups called clusters. One or more of the clusters are chosen and every member chosen is used. This is also very cheap but doesn’t remove all bias.

6. Control groups
A group of people are chosen at random to take part in a test, for example, patients for a new drug. The population is then split in two at random. Both groups think that they are getting the drug, however, only one group receives the drug. The group without the drug is called the control group. There may be more than two groups.
This can be used to see if something has an effect or if the problem is to do with the person.