Which of the following is an advantage of systematic sampling over random number sampling?
Data is gathered on a small part of the whole parent population or sampling frame, and used to inform what the whole picture is like Show
Why sample?In reality there is simply not enough; time, energy, money, labour/man power, equipment, access to suitable sites to measure every single item or site within the parent population or whole sampling frame. Therefore an appropriate sampling strategy is adopted to obtain a representative, and statistically valid sample of the whole. Sampling considerations
Sampling techniquesThree main types of sampling strategy:
Within these types, you may then decide on a; point, line, area method. Random sampling
The function is simply: Type that into a cell and it will produce a random number in that cell. Copy the formula throughout a selection of cells and it will produce random numbers. You can modify the formula to obtain whatever range you wish, for example if you wanted random numbers from one to 250, you could enter the following formula: Where INT eliminates the digits after the decimal, 250* creates the range to be covered, and +1 sets the lowest number in the range. Paired numbers could also be obtained using; These can then be used as grid coordinates, metre and centimetre sampling stations along a transect, or in any feasible way. MethodologyA. Random point sampling
B. Random line sampling
C. Random area sampling
Advantages and disadvantages of random samplingAdvantages:
Disadvantages:
Systematic samplingSamples are chosen in a systematic, or regular way.
MethodologyA. Systematic point sampling A grid can be used and the points can be at the intersections of the grid lines, or in the middle of each grid square. Sampling is done at the nearest feasible place. Along a transect line, sampling points for vegetation/pebble data collection could be identified systematically, for example every two metres or every 10th pebble B. Systematic line sampling The eastings or northings of the grid on a map can be used to identify transect lines. Alternatively, along a beach it could be decided that a transect up the beach will be conducted every 20 metres along the length of the beach C. Systematic area sampling A ‘pattern' of grid squares to be sampled can be identified using a map of the study area, for example every second/third grid square down or across the area - the south west corner will then mark the corner of a quadrat. Patterns can be any shape or direction as long as they are regular. Advantages and disadvantages of systematic samplingAdvantages:
Disadvantages:
Stratified samplingThis method is used when the parent population or sampling frame is made up of sub-sets of known size. These sub-sets make up different proportions of the total, and therefore sampling should be stratified to ensure that results are proportional and representative of the whole. A. Stratified systematic sampling The population can be divided into known groups, and each group sampled using a systematic approach. The number sampled in each group should be in proportion to its known size in the parent population. For example: the make-up of different social groups in the population of a town can be obtained, and then the number of questionnaires carried out in different parts of the town can be stratified in line with this information. A systematic approach can still be used by asking every fifth person. B. Stratified random sampling A wide range of data and fieldwork situations can lend themselves to this approach - wherever there are two study areas being compared, for example two woodlands, river catchments, rock types or a population with sub-sets of known size, for example woodland with distinctly different habitats. Random point, line or area techniques can be used as long as the number of measurements taken is in proportion to the size of the whole. For example: if an area of woodland was the study site, there would likely be different types of habitat (sub-sets) within it. Random sampling may altogether ‘miss' one or more of these. Stratified sampling would take into account the proportional area of each habitat type within the woodland and then each could be sampled accordingly; if 20 samples were to be taken in the woodland as a whole, and it was found that a shrubby clearing accounted for 10% of the total area, two samples would need to be taken within the clearing. The sample points could still be identified randomly or systematically within each separate area of woodland. Advantages and disadvantages of stratified sampling Advantages:
Disadvantages:
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