types of sampling Flashcards
(27 cards)
why are sampling techniques used
its impossible to count or measure all of the organisms present in an area
what is sampling
taking measurements of a limited number of individuals or organisms present in a particular area
what can sampling be used for
- estimating the number of organisms in an area without having to count them all.
- to measure a particular characteristic of an organism e.g. height - you can measure the height of a number of plants and then calculate an average
what is abundance
the number of individuals of a species
what do you do after measuring a sample
you can use results of the sample to make generalisations about the number of organism, distribution of species or measured characteristics throughout the entire habitat
what is random sampling
selecting individuals by chance. in a random sample, each individual in the population has an equal likelihood of selection
what is the process of random sampling
to decide which organism to study, random number tables or computers can be used. you have no involvement in deciding which organisms to investigate
1) mark out a grid on the grass using two tape measures laid at right angles
2) use random numbers to determine the x coordinate and the y coordinate on your grid
3) take a sample at each of the coordinate pairs generated
what are the advantages of random sampling
ensures data is not biased by selective sampling
what are the disadvantages of random sampling
may not cover all areas of a habitat equally
species with a low presence may be missed leading to an underestimate of biodiversity
what is non random sampling
an alternative sampling method where the sample isnt chosen at random
what is opportunistic sampling
weakest form of sampling as it may not be representative of the population. they use organisms that are conveniently available
advantages of opportunistic sampling
easier and quicker
disadvantages of opportunistic sampling
bias. can lead to overestimation of biodiversity
what is stratified sampling
some populations can be divided into a number of strata (subgroups) based on a particular characteristic. a random sample is taken from each of these strata proportional to size
advantages of stratified sampling
ensures all different areas of a habitat are sampled and species arent underrepresented due to the possibility that random sampling misses certain areas
disadvantages of stratified sampling
may lead to over representation of some areas in the sample
equation of stratified sampling
number selected from each strata =
(strata size/total population) x sample size
what is systematic sampling
different areas within an overall habitat are identified and sampled separately at specific points. often carried out using a line or belt transect
what is a line transect
involves marking a line along the ground between two poles and taking samples at specified points, this can include describing all the organisms which touch the line or distances of samples from the line
what is a belt transect
provides more information - two parallel lines are marked and samples are taken of the area between the two areas
advantages of systematic sampling
useful when the habitat shows a clear gradient in some environmental factor
disadvantages of systematic sampling
only species on the line or within the belt can be recorded. other species may be missed leading to an underestimate of biodiversity
why is a sample never entirely never representative of the organisms present in a habitat
due to sampling bias and/or chance
what is sampling bias
the selection process may be biased. this may be by accident or may occur deliberately.
