What is accuracy?
how close you observed or measured value is to the correct/real value
How can you improve accuracy?
Give an example
changing the experimental technique
e.g. ensure you use the most appropriate size of quadrat in transect measurements. Applies to any equipment really
What is reliability?
the consistency in (or your confidence with) a set of measurements
How can you improve reliability?
repetition - if repeated results lie close together then the results have high reliability
- taking repeat measurements also helps you identify and omit anomalous results
How can you comment on reliability?
- considering spread of results
- considering the closeness of replicates to the mean
- calculating variance, standard deviation, standard error or 95% confidence intervals
- considering the size of range bars or error bars on graphs
What is validity?
whether a test actually measures what it sets out to measure
How can you comment on validity/improve validity?
- identify factors that are not or cannot be controlled and which may therefore be limiting the validity of the investigation
What equipment can be used to measure water turbidity?
Secchi disk
Turbidity tube
Spectrophotometer
How does a Secchi disk work?
- an 8 inch black and white disk attached to a string with measurements on and a weight on the bottom
- it is lowered into the water until you cannot see the pattern any more
- you measure the depth that you can no longer see it
- values that are higher indicate clearer water with a lower concentration of suspended solids, lower values indicate high turbidity with a higher concentration of suspended solids
- this value is converted into NTU (Nephelometric Turbidity Unit)
- low NTU = low turbidity
How does a turbidity tube work?
- a clear tube with measurements and a cross/Secchi disk pattern painted in the bottom
- the sample liquid is added to the tube until the cross on the bottom of the tube is no longer visible
- the depth of the sample is then measured to the closest centimetre/millimetre
- this value is converted into NTU (Nephelometric Turbidity Unit)
- low NTU = low turbidity
How does a spectrophotometer work?
- its a piece of equipment that shines a beam of light and captures the amount of light that reflects back/scatters or transmits through to measure light penetration
- more scattering/reflection means that the light must have hit something, indicating high turbidity
What does Chi Squared measure?
is the data we have collected what we would expect or is it significantly different?
What is the Chi Squared formula?
χ2 = ∑(O – E)2/E
Outline one method that may be used to determine the percentage organic matter content of a soil sample?
- Dry soil sample
- Weigh it
- Burn it in a crucible to burn off the organic matter content
- Weigh it again
- Take the burnt weight from the original and convert it into a percentage
Outline a plan to investigate how wind speed changes with increasing distance from a stone wall
Anemometer on a transect
Repeat at different distances along the transect
Same heights, same time of day
Use a calibrated device
Repeat at least 10 times
How can we comment on significance?
“Significant at the x level, not significant at y. Can be more than x confident that the result did not occur by chance, so the null hypothesis is rejected at x confidence level”
OR
“…. can be x confident that the result did occur by chance, so the null hypothesis is accepted at x confidence level”
