Practical Terminology

Question 1

Accuracy

Answer 1

• A measure of the closeness of agreement between an individual test result and the true value
• If a test result is accurate, it is in close agreement with the true value
• An accepted reference value may be used as the true value, though in practice the true value is usually not known

Question 2

How to increase the accuracy of an experiment? (how can methodology be amended?)

Answer 2

• Repeat the experiment but suggest the range and interval (informed by first set of results), i.e. smaller range and smaller intervals
• Repeat the entire experiment where applicable
• Increase the number of samples to make it more 𝘳𝘦𝘱𝘳𝘦𝘴𝘦𝘯𝘵𝘢𝘵𝘪𝘷𝘦 (can be used for sampling using quadrats)

Question 3

Dairy famers need the land used for grazing by their cows to be as free from weeds as possible.

In the UK, dock plants are the most common perennial weed in grassland grazed by dairy cows.

Dock seeds are able to pass through the digestive tract of cattle unharmed. Cattle do not graze near cowpats so dock plants survive and grow in abundance.

Nettles can be found in plant material fed to cattle and these also survive passage through a cow’s digestive system. The plant chickweed grows well in soils with high nitrogen. Other plants commonly found in grassland are rye grass and white clover, as these are present in grass seed mix sown by farmers.

Explain how using a point quadrat could affect the accuracy of data collected (2)

Answer 3

• It only records what it hit/touched (1)
• Could be less representative (as less area covered) (1)
• May miss plants even though they are abundant (1)

Question 4

Validity

Answer 4

• A measurement is valid if it measures what is supposed to be measuring
• A method/procedure is valid if it is suitable to answer the question being asked
• Do not describe it as a ‘fair test’

Question 5

Validity can be reduced if:

Answer 5

• There is no negative control included in an investigation into the efficacy (the ability to produce a desired or intended result) of a therapeutic drug
• There are uncontrolled variables
• There is bias

Question 6

What do controls allow? (how do they affect the validity (of an experiment)?)

Answer 6

• A control allows comparable results, which proves the validity of an experiment
• e.g. respiration experiment with maggots and glass beads (control). The use of the control has made the experiment more valid, as the control is not producing results, rather, the maggots are

Question 7

The readings were used to plot a calibration curve for absorbance against glucose concentration. The student then carried out the same procedure on a sample of the sports drink. A value of 0.45 mol dm⁻³ was obtained for the sports drink from the calibration curve. The student concluded that the sports drink contained 0.45 mol dm⁻³ glucose.

Suggest one reason why the conclusion was not valid (1)

Answer 7

𝘐𝘥𝘦𝘢 𝘵𝘩𝘢𝘵 Benedict’s test is for reducing sugars/not specific to glucose

Question 8

What words should be used in place of ‘reliable/reliability’?

Answer 8

Repeatable and reproducible

Question 9

Repeatability

Answer 9

• The precision obtained when measurement results are produced over a short timescale by one person (or the same group) using the same equipment in the same place (so, same outcomes produced 3 times. If not, not a repeatable method)
• Range bars can be used to show variability in results. Longer bars = more variability and less repeatability

Question 10

Reproducibility

Answer 10

The precision obtained when measurement results are produced over a wider timescale by different people using equipment in different (but equivalent) places, i.e. same pattern of results

Question 11

Confidence

Answer 11

• The extent to which a conclusion is justified by the quality of the evidence
• Can be used to replace the word ‘reliable’
• i.e. higher confidence that results aren’t due to chance (reduced probability)

Question 12

A student used the following procedure to test different organs from a tomato plant for the presence of sucrose. Table 5 shows the observations recorded by the student:

Plant Organ Being Tested || 𝘖𝘣𝘴𝘦𝘳𝘷𝘢𝘵𝘪𝘰𝘯𝘴
Leaf || 𝘉𝘭𝘶𝘦-𝘨𝘳𝘦𝘦𝘯
Stem || 𝘎𝘳𝘦𝘦𝘯-𝘰𝘳𝘢𝘯𝘨𝘦
Root || 𝘉𝘭𝘶𝘦-𝘨𝘳𝘦𝘦𝘯

Method:

1. Remove a leaf from the tomato plant and after dipping in into boiling water, grind it using a mortar and pestle
2. Add water to the ground up leaf and filter the mixture
3. Pour a small sample of the filtrate into a test tube and add dilute HCl
4. Place the test tube into a water bath
5. Remove the test tube from the water bath and add sodium hydrogen carbonate
6. Add Benedict’s reagent and then place the test tube back into the water bath
7. Record the colour of the contents of the test tube
8. Repeat steps 1-7 with stem and root samples taken from the same tomato plant

State 3 modifications to the procedure that would allow the observations in table 5 to be reproducible (3)

Answer 12

Any three from:

• Same temperature of water bath
• (equal) volume of Benedict’s/test solutions
• Excess sodium hydrogen carbonate needed
• Same time left in water bath
• Use methods for obtaining quantitative results

Question 13

Why do we use repeats?

Answer 13

They allow us to:

• Identify anomalies
• Calculate a mean
• Assess variability in results
• Carry out a statistical test
• Improve the reliability, so that the experiment is repeatable

Question 14

Anomaly/outlier

Answer 14

A value in a set of results that is judged not to be part of the inherent variation

Question 15

How many repeats of an experiment should be done?

Answer 15

2, so that 3 sets of results are taken

Question 16

Precision

Answer 16

• If repeats are the same than the precision level is high (the closeness between independent measurements obtained under the same conditions)
• Depends on the spread of measurements and does not relate to the true value