Data collection

What is a population?

The entire set of items (sampling units) that are of interest

What is a census?

An observation or measurement from every member of a population

What are the advantages and disadvantages of taking a census?

ADV: Should give a completely accurate result.

DISADV: 1. Time consuming for large populations

2. expensive for large populations

What is a sample?

A selection of observations or measurements from a subset of the members of a population

What are the advantages and disadvantages of using a sample?

ADV: 1. Less time-consuming than a census

DISADV: 1. Doesn't measure every member of the population — inaccurate and not fully representative

2. Sample size might not be large enough to represent the entire population

What does the size of a sample depend on?

The required accuracy and available resources

Generally

what creates a more accurate sample?

If the population is varied

what do you need?

What can different samples lead to?

Different conclusions due to the natural variation in a population

What can the size of a sample affect?

The validity of any conclusions drawn

What is a sampling unit?

Individual units of a population (e.g. one single avocado)

What is a sampling frame?

A named or numbered list of each sampling unit in a given population

What does random sampling help to remove?

Bias

What are the three methods of random sampling?

1. Simple random sampling

2. Systematic sampling

3. Stratified sampling

What is a simple random sample and how is one taken?

Items from the sampling frame are selected at random

What is a simple random sample of size n?

One where every sample of size n has an equal chance of being selected

What are the advantages (2) and disadvantages(3) of simple random sampling?

ADV: 1. Bias free

DISADV: 1. Sampling frame required

2. Time-consuming for large samples

What is systematic sampling and how is it carried out?

All items of the sampling frame are ordered

What are the advantages (2) and disadvantages (2) of systematic sampling?

ADV: 1. If sampling frame is randomly ordered

DISADV: 1. Sampling frame required

2. Non-randomly ordered data may introduce bias

What is stratified sampling?

• The population is first divided into mutually exclusive strata (groups/categories) and the number of members in each category is noted.

• The sample is then made up of a proportionally representative number of members to reflect the population.

What are the advantages (3) and disadvantages (2) of stratified sampling?

ADV: 1. Represents population structure

DISADV: 1. Sampling frame needed

2. Need clear strata/categories/groups in the population

How do you calculate the number sampled in a stratum?

(Number in stratum/number in population) x overall sample size

What are two methods of selecting numbers for random sampling?

1. Generating random numbers using a calculator

2. Lottery sampling

What happens in lottery sampling?

The members of the sampling frame could be written on tickets and placed into a 'hat'. The required number of tickets would then be randomly pulled out

What are two types of non-random sampling? What is their advantage?

1. Quota sampling

2. Opportunity sampling

No sampling frame required

What is quota sampling?

1. Researcher splits population into groups

2. A quota to be filled is noted down for each group (this may or may not be representative of the population).

3. The quotas are then filled using opportunity sampling. If any member is selected for a quota that has already been filled

they are ignored and move on.

What are the advantages and disadvantages of quota sampling?

ADV: 1. No sampling frame needed

DISADV: 1. Non-random sampling can introduce bias

2. Population must be divided into groups which can be difficult or costly or inaccurate

How do you determine a suitable quota?

• use electoral register to determine the size of each group as a proportion of the whole population

• assign the quotas as the same proportion of the whole sample

What is opportunity sampling?

Consists of 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

What are the advantages and disadvantages of opportunity sampling?

ADV: 1. Easy and cheap to carry out

DISADV: 1. Unlikely to provide a representative sample

2. Results can vary depending on the individual researcher

What are quantitative variables/data?

Numerical data

What are qualitative variables/data?

Non-numerical data

What is a continuous variable?

A variable that can take any value in a given range

What is a discrete variable?

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

What do class boundaries tell you?

The maximum and minimum values that belong in each class

What does the midpoint tell you?

The average of the class boundaries

What does the class width tell you?

The difference between the upper and lower class boundaries

What is the large data set?

Weather data samples provided by the Met Office for 5 U.K. weather stations and 3 overseas

What is the daily mean temperature?

The average of the hourly temperature readings during a 24 hour period

What is the daily total rainfall?

including solid precipitation such as snow and hail

What is daily total sunshine?

Recorded to the nearest tenth of an hour

What is daily mean wind direction and wind speed?

In knots

What is daily maximum gust?

The highest instantaneous windspeed recorded - measure in knots

What is daily maximum relative humidity?

Percentage of air saturation with water vapours. A continuous variable that can take any value within 0-100.

What is daily mean cloud cover?

Measured in oktas or eighths of the sky covered by cloud. Cannot be higher than 8.

What is daily mean visibility?

Measured in decametres (Dm). This is the greatest horizontal distance at which an object can be seen in daylight

what is daily mean pressure?

Measured in hectopascals (hPa)

Why might a median be less than a mean?

• the distribution is skewed

• a few large distances distort the mean

Why would you use the median and interquartile range rather than the mean and standard deviation?

If the data is skewed as mean and standard deviation are affected by extreme values