Year 13 EMPA advice

Problems year 12 had with task 1 pectin expt

• Repeating readings makes the mean more reliable. Don’t say that the results are more reliable.

• Accuracy cannot be improved by using a stopwatch that measures to 0.0001s because the main source of error is the time taken to start and stop it, and to determine the end point of the syringe emptying.

• Using a bigger syringe will make the readings more accurate because the time for the syringe to empty is longer. This means that the error time to start or stop will be a smaller % of the total time taken.

EMPA Task 1

• Is a preliminary investigation

• Has written questions associated with it

• Typically worth 8 marks

For AS EMPARecording data in a table

- column heading requires a full description of variable

- independent variable in the first column

- units only in headings

- units do not change / are not mixed

- unit separated from the variable by a solidus (but brackets accepted)

- a title is good practice (but not assessed as such) because it can help an incomplete label

Assessing the table - 1

Investigation example produces data of time taken for pH indicator to turn blue following exposure of respiring bacteria to different temperatures

What is wrong with the following tables?

-

Table 1

Temperature (oC)

Test 1 Test 2 Test 3 Average (m)

Colour Time (m)

Colour Time (m)

Colour Time (m)

20 Blue 3.20 Blue 1.55 Blue 2.50 2.41

30 Blue 2.53 Blue 3.06 Blue 4.09 3.22

40 Blue 1.26 Blue 2.35 Blue 2.20 1.93

50 Blue 1.46 Blue 1.02 Blue 0.45 0.97

60 Blue 0.60 Blue 1.25 Blue 2.26 1.37

Table 3

Test tube Temperature of water bath

Time taken to turn blue

Repeat time taken to turn blue

Average

1 20 oC 7 minutes 15 seconds

7 minutes 11 seconds

7 minutes 13 seconds

2 30 oC 2 minutes 1 minute 45 seconds

1 minute 52.5 seconds

3 40 oC 36 seconds 47 seconds 41.5 seconds

4 50 oC 34 seconds 31 seconds 32.5 seconds

5 60 oC 8 minutes 57 seconds

9 minutes 33 seconds

9 minutes 15 seconds

Processing data

Students should be able to calculate

• Mean

• Standard deviation

• Rate

• Percentage change

Preparing Students – Drawing a graph - requirements

• Independent variable and dependent variables on correct axes

• Appropriate use of scale• Labels on axes complete with appropriate

units• (Mean) data plotted accurately• Appropriate line• Correct choice of line graph, bar chart,

histogram or scatter diagram

Drawing a graph – expectations 1

• IV (x-axis) and DV (y-axis)• Appropriate use of scale

- to allow accurate plotting and reading off of values- scale is linear- axes can start at values above zero

• Labels of both axes complete with appropriate units- a title is good practice (but not assessed as such) because it can help an incomplete label

• All (mean) data plotted accurately- choose a scale that is suitable for plotting

Drawing a graph – expectations 2

• Appropriate line- plotted points joined- line of best fit is a smooth curve or a straight (ruled) line- line of best fit reflects plotted data- point to point (ruled) lines are acceptable- no extrapolation (back to zero or beyond plotted values)

Drawing a graph – expectations 3• Correct choice of line graph, bar chart, histogram or

scatter diagram- line graph where data for both IV and DV are continuous- bar chart where IV is categoric (presented as lines or blocks of equal width which do not touch) and DV is continuous- histogram where IV is continuous (presented as blocks which do touch) and DV represents frequencies- scatter diagram used when investigating relationship between two naturally changing variables

HSW glossary - 1

• Accurate refers to a measurement is one which is close to the true value.

• Anomalous data are those measurements that fall outside the normal, or expected, range of measured values.

• A Control experiment is one that is set up to show that the (named) independent variable is responsible for the change

HSW glossary - 2

• Results of an investigation are Reliable if they can be repeated

• Data are Valid if the measurements that have been made are affected by a single independent variable only

• Conclusions are only valid if they are supported by valid and reliable data measured to an appropriate level of accuracy.

• (One to avoid? – use of ‘fair test’)

Some other examples

• Why repeat an experiment or investigation?- to allow anomalous data to be identified (with greater certainty)- to produce a mean value that is reliable

• Why are the sizes of pieces of potato kept the same / doses of an experimental drug given per kg body mass of patients?- to allow a comparison

Validity, Reliability, Accuracy – which?

• ‘ I repeated the investigation so that the calculated mean would be more……….’

• ‘I used a pipette instead of a measuring cylinder to improve the ……… of the measurement.’

• ‘Based on the data I have collected I think my conclusion is ……….’

• ‘There are no identifiable anomalies so I think the data are ………..’

• ‘It is not ……. to say that the blood pressure of all adults is the same’

• ‘ I repeated the investigation so that the calculated mean would be more reliable’

• ‘I used a pipette instead of a measuring cylinder to improve the ……… of the measurement.’

• ‘Based on the data I have collected I think my conclusion is ……….’

• ‘There are no identifiable anomalies so I think the data are ………..’

• ‘It is not ……. to say that the blood pressure of all adults is the same’

• ‘ I repeated the investigation so that the calculated mean would be more reliable’

• ‘I used a pipette instead of a measuring cylinder to improve the accuracy of the measurement.’

• ‘Based on the data I have collected I think my conclusion is ……….’

• ‘There are no identifiable anomalies so I think the data are ………..’

• ‘It is not ……. to say that the blood pressure of all adults is the same’

• ‘ I repeated the investigation so that the calculated mean would be more reliable’

• ‘I used a pipette instead of a measuring cylinder to improve the accuracy of the measurement.’

• ‘Based on the data I have collected I think my conclusion is valid’

• ‘There are no identifiable anomalies so I think the data are ………..’

• ‘It is not ……. to say that the blood pressure of all adults is the same’

• ‘ I repeated the investigation so that the calculated mean would be more reliable’

• ‘I used a pipette instead of a measuring cylinder to improve the accuracy of the measurement.’

• ‘Based on the data I have collected I think my conclusion is valid’

• ‘There are no identifiable anomalies so I think the data are reliable.’

• ‘It is not ……. to say that the blood pressure of all adults is the same’

• ‘ I repeated the investigation so that the calculated mean would be more reliable’

• ‘I used a pipette instead of a measuring cylinder to improve the accuracy of the measurement.’

• ‘Based on the data I have collected I think my conclusion is valid’

• ‘There are no identifiable anomalies so I think the data are reliable.’

• ‘It is not valid to say that the blood pressure of all adults is the same’

Preparing Students – Following the instructions

• Understand what the term expects e.g. describe, explain, suggest etc.

• ‘Use the data’ or ‘Use a calculation to support your answer’ – so quote figures

• What are the advantages and disadvantages…? – so give both

• ‘Give two reasons to….’ – giving one cannot achieve full marks

• ‘Give the result of your experiment…’ - “There was no change” is ambiguous but, for example, “it remained blue” is not

Preparing Students – Recognising limitations in investigations

(or the converse)

• Anomalies present

• Sample size small

• Lack of replicates

• (Identified) variables not controlled

• Confounding variables present

• Interpretation is subjective

• Results do not prove a causal link

Preparing Students – Recognising limitations in investigations

• General principle

The work of a ‘scientist’ should be considered sound. In this case the limitations would be different to an investigation carried out by ‘another student’.

e.g. “you don’t know if they used the same number of animals in their trials” – you would expect them to so it is not a limitation.

Preparing for A2

Graph type? (1)• Effect of pH on rate of

hydrolysis of starch by amylase

• Area of stain removed by lipases in different washing powders

• Effect of posture on heart rate

• Effect of salt concentration on diameter of onion rings

• Height of all male students within the class

- worksheet

• Line graph

• Bar chart

• Bar chart

• Line graph

• Histogram

Graph type? (2)• Percentage cover of

dandelions on different football pitches

• Frequency of blood groups within the population

• Distribution of number of stamens per flower in a population of buttercups

• Effect of different mineral ions on germination of lettuce seeds

• Bar chart

• Bar chart

• Histogram

• Bar chart

Graph type? (3)

• Volume of irrigation water applied and yield of wheat from field

• Ages of runners and times to complete 400m

• Masses of fathers and their sons

• Scatter diagram

• Scatter diagram

• Scatter diagram

Progression from AS – essential differences at A2 (1)

• Questions a little more demanding• Fewer signposts

‘In terms of water potential explain why a potato cylinder loses mass when placed in a salt solution’ – AS

‘The mass of a potato cylinder will change when placed in a salt solution. Explain how’ – A2

Progression from AS – essential differences at A2 (2)

• Task is likely to have a synoptic element- practical and investigative skills from AS

- experience from use of apparatus and techniques- experience from required investigations- choice of graph

• Written Paper Section B- ability to link information from several or all

Resources expected

Linking information – progression at A2

• ‘A journalist wrote an column in a newspaper claiming that using liquid fertilisers improved the taste of GM cucumbers. Use the resources to evaluate this statement.’ – A2

• (final sentence for AS might read ‘Using Resources C and D, give the evidence for and against this claim.’)

Progression from AS – essential differences at A2 (3)

• Processing requires statistical treatment of data- enough data - null hypothesis stated- appropriate statistical test chosen - reasons for choice of the test - calculation of test statistic made accurately- test statistic interpreted

degrees of freedom probability levelcritical valueaccept or reject null hypothesischance

Practical tasks we have done• Investigate populations

- study a habitat- use frame quadrats and line transects- measure an abiotic factor- measure abundance (population density) by percentage cover and frequency

• Photosynthesis- effect of limiting factor (light intensity / CO2 concentration / temperature) on rate

• Respiration- effect of substrate / temperature on rate in an organism

• Taxes and Kineses- the effect of external stimuli

A2 practical / investigative skills we have done

Skill

• Random sampling• Use of 3-way tap• Establishing

anaerobic conditions

Opportunity

- Measure abundance

- Respiration in yeast

- Respiration in yeast

Synopsis

• Transfer of practical and investigative skills from AS during Tasks

• Equipment use e.g. knowledge of apparatus• Methodology e.g. use of a control• Terminology e.g. reliable, valid etc.• Table skills still required• Identifying type of graph to use still possible • Calculations still expected (of rate, mean,

standard deviation and percentage change)• Biological ideas brought forward e.g. enzyme

action.

Some things candidates are not good at!• Reading the question stem

- it provides the focus or context for the answer• Answering the question asked• Using language that is helpful

- “It will change / effect / alter …..” But how? What is ‘it’?• Recognising the number of marks available• Defining ‘Control’

- What is a suitable control? The same volume is needed so something must be replaced- Why is a control used? To show the independent variable is responsible for any change

• Describing what standard deviation bars show and explaining their use - if they do not overlap differences are real- the longer the bar the greater the variation in or spread of data

Practice EMPA

• In this example there is no task 1

• We will be investigating the distribution of Pleurococcus on the bark of trees

How will you randomly sample?

How will you use a quadrat?

Some sample data

• Your teacher will give you a sheet with some sample data for you to work on