data collection

Question 1

Population

Answer 1

• The whole set of items that are of interest .

Question 2

Census

Answer 2

-Observes or measures every member of the population.

Question 3

Sample

Answer 3

-A selection of observations taken from a subset of the population to represent the population.

Question 4

Advantages of Census.

Answer 4

• Gives completely accurate results.

Question 5

Disadvantages of a Census.

Answer 5

• Time consuming and expensive.
• Hard to process a large quantity of data.
• Can’t be used when a testing process destroys the item.

Question 6

Advantages of a Sample

Answer 6

• Less time consuming and expensive than a census.
• Less data to process than a census.

Question 7

Disadvantages of a Sample

Answer 7

• Data may not be as accurate.
• The sample may not be large enough to give information about small sub-groups of a population.

Question 8

How can the size of the sample affect the validity of any conclusions drawn ?

Answer 8

• Depends on the required accuracy and the availability of resources.
• Very varied population = larger sample size.
• Different samples can lead to different results due to the natural variation in a population.

In conclusion:
THE LARGER THE SAMPLE IS = THE MORE ACCURATE THE RESULTS .

Question 9

Sampling units

Answer 9

• Individual units in a population.

Question 10

Sampling frame

Answer 10

• A list of sampling units that are individually named or numbered.

Question 11

Simple random sampling

Answer 11

• A simple random sample of size (n) is one where every sample of size (n) has equal chance of being selected ; removing any bias.
• Steps : - A sampling frame is needed , eg. a list of people or things
  - Each person or thing is allocated a unique number and a selection of these numbers is chosen at random , either by
  a random number generator or lottery sampling.
• Advantages : - Free of bias
  - Easy and cheap to implement for small populations and samples.
• Disadvantages : - A sampling frame is needed
  - Not suitable when population/sample size is large as it can be time consuming,disruptive and expensive.

Question 12

Systematic sampling

Answer 12

• Required elements are chosen at regular intervals from an ordered list .
• However the first person to be chosen is picked at random.
• Advantages : - Simple and quick to use .
  - Suitable for large populations and samples .
• Disadvantages : - Requires a sampling frame .
  - It can introduce bias if sampling frame is not random .

Question 13

Stratified sampling.

Answer 13

• The population is divided into mutually exclusive strata and a random sample is taken from each .
• THE NUMBER SAMPLED IN STRATUM = NUMBER IN STRATUM / NUMBER IN POPULATION * OVERALL SAMPLE SIZE
• Steps : - Find the number of workers in each age group using the equation above.
  - Number the workers in each age group.
  - Use a random number generator to produce the required quantity of random numbers.
  - Give the questionnaire to the workers corresponding to these numbers.
• Advantages : - Sample accurately reflects the population structure .
  - Guarantees proportional representation of groups within a population .
• Disadvantages : - Population must be clearly classified into distinct strata .
  - A sampling frame is needed .
  - Not suitable when population size is large .

Question 14

Quota sampling

Answer 14

• An interviewer or researcher selects a sample that reflects the characteristics of the whole population.
• The population is divided into groups according to a given characteristic. The size of each group determines the proportion of the sample that should have the characteristic.
• As an interviewer you would : - meet people
  - assess their group
  - allocate them into the appropriate quota/strata
• If a person refuses to be interviewed or the quota into which they fit is full, then you simply ignore them and move on to the next person.
• Advantages : - No sampling frame is requires
  - Quick,easy and inexpensive
  - Allows for easy comparison between different groups within a population.
• Disadvantages : - Can introduce bias
  - Population must be divided into groups , which can be costly or inaccurate
  - Non-responses are not recorded

Question 15

Opportunity sampling

Answer 15

• Taking the sample from people who are available at the time the study is carried out and who fit the criteria you are looking for.
• You continue to sample individuals into a sample until you make the sample size you need.
• Advantages : - Easy to carry out
  - Inexpensive
• Disadvantages : - Unlikely to provide a representation sample.
  - Highly dependent on individual researcher.

Question 16

Continuous variable

Answer 16

• A variable that can take any value in a given range , eg. time and temperature.

Question 17

Discrete variable

Answer 17

• A variable that can only take specific values in a given range .