Unit 3 Investigative Biology

SQA

Success Criteria  Explain the difference between random sampling, systematic sampling and stratified sampling.

Sampling  When collecting data about a population, it is obviously ideal to measure and make a record of every individual.  However, unless the whole population is very small this is not practical!  To overcome this, researchers measure a representative sample of the population.  This is called sampling. Obviously...

Sampling All populations show natural variation More variable populations will require a larger sample size Less variable populations can be measured effectively with a small sample

What is a representative sample? It should share the same mean as the population as a whole It should share the same variation about the mean as the population as a whole Sampling

Random Sampling  In random sampling all members of the population have an equal chance of being selected.  Random sampling is usually used when the population being tested is very large or uniform or if there is a limited timeframe in which to obtain results.  Using random selection of samples is important as statistical analysis is only effective when sampling is truly random.

Random Sampling  Using a random number generator overcomes the risk of non-random sampling.  The random numbers that are generated are applied to the population or area being measured.

Ecological sampling  Self-selection of sampling sites is rarely truly random.  Researcher bias can occur even when trying to be random.  One way to overcome this is by mapping the area to be sampled and using a numbered grid to map out the area.  Random number generators then used...

Systematic sampling Line transect  A line transect can be made by placing a piece of rope marked at 1 m intervals or along a measuring tape along the area you wish to sample.  Species that are touching the line can be recorded continuously along the line or at regular intervals.

Line transect

Belt transect  A belt transect is similar to a line transect but can give a measurement of species abundance as well as the presence/absence of species.  Quadrats are placed between two lines (or on either side of the line) and species recorded at regular intervals along the line.

Stratified sampling  When sampling populations or habitats, often they are not uniform.  In a marine environment, a transect through the tidal zones on a beach may have areas of bare rock as well as algae.  Random sampling may result in a disproportionate emphasis being placed on one type of ground cover if the sampling is always selected on grass or bare rock.  To overcome this, stratified sampling is used.  The population is divided into categories that are then sampled proportionally

Stratified sampling

Stratified sampling (2) Example of heath land, which is not uniform. Stratified sampling would allow the correct proportion of the areas that are grass, bush, small trees or large trees to be sampled. There is a standard formula for calculating the number of samples that should be allocated to each unit (eg grass, bush): number of quadrats sampled in the area = the area of the unit × the total number of quadrats to be sampled total area of the habitat

Unit 3 Investigative Biology

SQA

Success Criteria  Explain clearly the difference between reliability and accuracy.

Reliability  The reliability of procedures can be determined by repeated measurements or readings.  The variation observed indicates the precision of the measurement, instrument or procedure but not necessarily its accuracy.

Reliability versus Accuracy

Ensuring reliability in experiments Variation in experimental results The reliability of measuring methods Inherent variation in the specimens being measured

Reliability of measuring instruments Repeated measurements or readings of an individual data point increases reliability Reducing measurement variation indicates the precision of the instrument or its use but not necessarily accuracy Accuracy of measurement can be ascertained by calibrating results against a known standard Repeating measurements is only valid if the same experimenter is measuring results to increase reliability Measuring instruments

Natural variation of biological material Human heart rate The average human resting heart rate is between 60 and 100 bpm, but this varies across the population depending on age, sex, weight, level of fitness, illness and stress. Heart rate of water fleas (Daphnia ) The average heart rate of Daphnia can be up to 300 bpm. Note: It becomes physically more difficult for an experimenter to count heartbeats accurately after 220 per minute. Heart rate varies according to size of water flea and, most importantly, the temperature of the solution the Daphnia are kept in.

Natural variation of biological material

 The extent of natural variation in the biological material being used can be determined by measuring a sample of individuals from the population.  The mean of these repeated measurements will give an indication of the true value for the organism being measured.  Remember that when the natural variation of biological material is high, a larger sample size is necessary to obtain a reliable mean value.  Remember also to factor in the limitation of the method of measurement – trying to count Daphnia heart beats per minute becomes increasingly more difficult for an experimenter to do accurately above about 220 bpm.

Ensuring reliability of results Results can only be considered reliable if they can be achieved consistently Check consistency by repeating experiments Check consistency by pooling results or referencing scientific literature The experiment should be repeated as a whole to check the reliability of the results Ensuring reliability

Success Criteria  Explain the difference between random sampling, systematic sampling and stratified sampling.  Explain clearly the difference between reliability and accuracy.  Explain the difference between random sampling, systematic sampling and stratified sampling.  Explain clearly the difference between reliability and accuracy.