Presentation on theme: "SAMPLING Pages 14 to 20. SAMPLING What is it? -A sample is a small part or a fragment of a whole used to represent what the whole picture is really like."— Presentation transcript:
SAMPLING What is it? -A sample is a small part or a fragment of a whole used to represent what the whole picture is really like. -The whole picture is called the ‘sampling frame’ and comprises the total ‘population’ of items. -For example an area of woodland is the sampling frame and the total number of trees in it is the population. Why use sampling? Impossible, impractical and unnecessary to study the entire population. Sampling is an accepted short cut, it is easier, quicker and cheaper. For example if looking at how a river changes with distance downstream it is impossible to study the entire course of the river, so a number of carefully selected sites will be used which forms a sample.
How do you select a sample? There are 4 main things to consider: 1.Sample size 2.Elimination of bias 3.Type of sampling procedure 4.Type of sampling method The larger the size of the sample the more accurately it will reflect the total population. Standard deviation can be used to calculate the minimum sample size required, depending on how reliable you want your data to be. http://geographyfieldwork.com/Mi nimumSampleSize.htm 1. Sample Size
The Normal Distribution Curve Normal Distribution Curve ( Bell Curve) Key to graph RED = One standard deviation away from the mean in either direction on the horizontal axis (68% of population included) RED + GREEN = Two standard deviations away from the mean (95% of population included) RED + GREEN + BLUE = Three standard deviations away from the mean (99% of total population included) When using a sample you want to aim for the mean of the sample to closely equate with the mean of the total population. MEAN Frequency -3-2123 Large no. of values lie close to the mean Fewer values lie at the extremes Standard Deviations
2. Elimination of bias Bias needs to be eliminated if data collected is to be accurate and representative. To do this you need to carefully consider the characteristics of your population. For example if carrying out a survey on opinions regarding a new leisure centre you need to question people of all ages and economic groups. If you just did a sample in the most accessible areas, or the area you feel most comfortable with, your sample would be biased. To overcome the risk of bias different types of sampling procedures and methods must be used.
3. Type of Sampling Procedures A) Point SamplingB) Grid or Area Sampling Individual points, or sites, are selected for investigation. E.g. sites along a river, or a vegetation sampling quadrat which uses pins to locate points. Involves delimiting an area for study. The size of area can vary e.g. a grid quadrat for vegetation sampling.
Continued….. C) Line or Transect Sampling D) Belt Sampling A line is plotted along which data collection will occur. Generally used when the variable under investigation are expected to show progressive variation spatially. E.g. a housing survey along a road, or succession of vegetation long a line. An area along a line is defined for investigation. This could be used for vegetation or soil studies where there is a wider areal focus.
4. Type of sampling method A) RANDOM Used when area very large or limited time available. A numbered grid can be laid over a map of the area. A computer generated random number table is then used to select which squares to sample in.
Continued….. B) SYSTEMATIC Samples are taken at fixed intervals, usually along a line. Line or transect sampling and belt sampling fall into this category. Could be used to show how price buildings changes with distance from the CBD. C) STRATIFIED Used where there are small areas within a larger study location which are clearly different. E.g. an area with more elderly and very young people. The sample must include a representative proportion of the elderly and very young. Sampling could still be carried out randomly or systematically within each separate sub-group. Stratified sampling recognises major differences within communities before sampling begins.
Questions see pages 14 to 23 1. Identify 3 factors which need to be considered by a geographer when selecting a sample population for study. (June 2004) 2. Explain why systematic sampling would be used if investigating the changes in a river channel downstream. 3. I want to carry out an investigation in Dungannon about attitudes towards the builiding of a new gaelic pitch at a particular site in the town. The population of the town is 40% Protestant, 50% Catholic, & 10% other. What type of sampling would be best and why? 4. A mixture of Stratified and Random sampling was used to carry out a vegetation survey at Peatlands Park comparing the damage done by drainage and peat extraction. Explain how this may have been carried out and why this may have been the most appropriate method.
Questions continued….. 5. What does the normal distribution curve show? 6. In tabular format briefly outline the advantage and disadvantages of the 3 sampling methods. 7. What is a quadrat? 8. Give 3 reasons for using a quadrat in a vegetation study. 9. Explain the differences between the 3 types of quadrats and draw each. 10. Why might you want to alter the size of your quadrat? 11. What is a questionnaire? 12. Explain the 3 types of questions which can be used when designing a questionnaire. 13. Draw a spider to show the things you should consider when designing a questionnaire.