Sampling (Edexcel IGCSE Geography)

Purpose of Sampling

• It gives an overview of the whole feature/population to be sampled
• There is not enough time/equipment/access to measure the whole area being examined
• Sampling provides a representative and statistically valid sample of the whole

Types of Sampling

• There are three types of sampling to consider
  • Random
  • Systematic
  • Stratified
• Random sampling
  • A grid is drawn/placed over the area to be studied
  • The squares which include part of the study area are numbered
  • The numbers are entered into a random number generator 
  • The samples should be collected as near as possible to the points given
• Systematic sampling
  • The samples are selected at regular intervals for example every 500 meters or every tenth person
• Stratified sampling
  • Used when the study area includes significantly different parts known as subsets 
  • Is based on the idea that the sample represents the whole population 
  • If a questionnaire is being used to collect data and the population of the study area has 10% of people over 65, then the sample should include 10% of people over 65

• All sampling methods have advantages and disadvantages

Sampling type	Advantages	Disadvantages
Random	Least biased of all sampling all possible sample sites have an equal chance of being selected Can be used with a large sample area/population	Representation of the overall population may be poor if the random sites miss large areas  Some sites selected may not be accessible or safe
Systematic	It is easy and quick making it more straightforward than random sampling It covers the whole study area equally	Not all sites have an equal chance of being selected which increases the bias There may be over or under-representation of a particular feature
Stratified	It can be used alongside systematic and random sampling Comparisons can be made between sub-sets	The proportions of sub-sets need to be known and accurate