Biology PAG 3: Sampling techniques
Combined Science PAG B2: Sampling techniques
Suggested Activity 1: Capture-recapture
Instructions and answers for teachers &
technicians
These instructions cover the learner activity section which can be found on page 9. This Practical
activity supports OCR GCSE Biology.
When distributing the activity section to the learners either as a printed copy or as a Word
file you will need to remove the teacher instructions section.
This is a suggested practical activity that can be used as part of teaching the GCSE (9-1)
Gateway Science (A) and Twenty First Century Science (B) specifications.
These are not controlled assessment tasks, and there is no
requirement to use these particular activities.
You may modify these activities to suit your learners and centre. Alternative activities are
available from, for example, Royal Society of Biology, Royal Society of Chemistry, Institute of
Physics, CLEAPSS and publishing companies, or of your own devising.
Further details are available in the specifications (Practical Skills Topics), and in these videos.
OCR recommendations:
Before carrying out any experiment or demonstration based on this guidance, it is the
responsibility of teachers to ensure that they have undertaken a risk assessment in
accordance with their employer’s requirements, making use of up-to-date information and
taking account of their own particular circumstances. Any local rules or restrictions issued
by the employer must always be followed.
CLEAPSS resources are useful for carrying out risk-assessments:
(http://science.cleapss.org.uk).
Centres should trial experiments in advance of giving them to learners. Centres may
choose to make adaptations to this practical activity, but should be aware that this may
affect the Apparatus and Techniques covered by the learner.
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Introduction
The capture-recapture technique is designed to estimate the number of species in a population.
With the capture-recapture technique animals are caught in humane traps. These are marked in
some way (e.g. paint mark, ringing etc.) and then released. After a period of time then a second
sample is taken. The number of marked individuals found in the second sample is used to
calculate an estimate of the population size.
In the classic laboratory experiment, woodlice or a similar animal are used, either in the wild or in a
large container.
The problem with this approach is that the safe and ethical use of the animals cannot be
guaranteed. In the container the animals can often die, even with the best efforts of the laboratory
staff, as the container cannot exactly match the animals’ normal habitat. Marked individuals can be
easier to spot by predators; so the marking process can be a disadvantage to the animal.
This technique uses the learners’ peers and can be used to estimate the number of learners in the
centre. This can be compared with the known population of the centre and as such can be used to
evaluate the technique and to see how accurate the approach is.
DfE Apparatus and Techniques covered
6 [6]: Application of appropriate sampling techniques to investigate the distribution and abundance
of organisms in an ecosystem via direct use in the field.
Aims
To become familiar with the methods of sampling biotic and abiotic factors in a habitat
To collect data in a non-laboratory situation
To analyse data
To evaluate the effectiveness of sampling methods
Intended class time
This activity will take approximately 15 minutes to set up then a short period of time for sampling.
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Links to Specifications:
Gateway
B6.1a explain how to carry out a field investigation into the distribution and abundance of
organisms in a habitat and how to determine their numbers in a given area sampling techniques
(random and transects, capture-recapture), use of quadrats, pooters, nets, keys and scaling up
methods
Twenty First Century
B3.4.2. describe how to carry out a field investigation into the distribution and abundance of
organisms in an ecosystem and explain how to determine their numbers in a given area
B3.4.3. in the context of data related to organisms within a population:
a) calculate arithmetic means
b) use fractions and percentages
c) plot and draw appropriate graphs selecting appropriate scales for the axes
d) extract and interpret information from charts, graphs and tables
Mathematical Skills covered
Understand the principles of sampling as applied to scientific data
Find arithmetic means
Understand the terms mean, mode and median
Gateway Working scientifically references covered
WS1.3a presenting observations and other data using appropriate method to include descriptive,
tabular diagrammatic and graphically
WS1.3c carrying out and representing mathematical and statistical analysis statistical analysis to
include arithmetic means, mode, median
WS1.3h identifying potential sources of random and systematic error
WS1.3i communicating the scientific rationale for investigations, methods used, findings and
reasoned conclusions presentations through paper-based presentations using diagrammatic,
graphical, numerical and symbolic forms
WS2a carry out experiments due regard to the correct manipulation of apparatus, the accuracy of
measurements and health and safety considerations, and following written instructions
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WS2b make and record observations and measurements using a range of apparatus and methods
keeping appropriate records
WS2c presenting observations using appropriate methods to include descriptive, tabular
diagrammatic and graphically
WS2d communicating the scientific rationale for investigations, methods used, findings and
reasoned conclusions presentations through paper-based and electronic reports and presentations
using verbal, diagrammatic, graphical, numerical and symbolic forms
Twenty First Century IaS references covered
IaS1.5 suggest an appropriate sample size and/or range of values to be measured and justify the
suggestion
IaS2.1 present observations and other data using appropriate formats
IaS2.9. in a given context evaluate data in terms of accuracy, precision, repeatability and
reproducibility, identify potential sources of random and systematic error, and discuss the decision
to discard or retain an outlier
IaS2.10. evaluate an experimental strategy, suggest improvements and explain why they would
increase the quality (accuracy, precision, repeatability and reproducibility) of the data collected,
and suggest further investigations
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Essential equipment (per group)
Set up:

Coloured labels

A group of people to sample e.g. another class, nurses etc.
Method
Set up:

This technique is practically simple but it does need a sample to test. The sample must not
know why they are being used as this will skew the result.

Chose a non-GCSE class e.g. a year 7 class.

Inform the learners in the test class that they are involved in a scientific experiment and that
they must not remove the coloured label until after they leave the site.

Record the number of students that were labelled.

Decide on a suitable time and location to resample. This could be break
time/lunchtime/home time and the tuck shop/dinner queue/centre entrance.

At the chosen time and location record a suitable sample e.g. 100 individuals in total

Identify the number within that sample that are labelled.
Notes
The use of coloured labels means that multiple classes can do the same experiment at the same
time. Learners from a class will only need to record the individuals in their sample that are labelled
with their colour. If more than one class is doing the experiment at a time the results can be
averaged or compared.
This approach can also potentially be used in hospital schools, replacing the sample class with
ward nurses for example. The learner could then look at the number of nurses in the staff canteen
and the number that are labelled. They can use the data to estimate the number of nurses in the
hospital (data that could be found from the senior nurse/administrator).
Technician Notes
For this practical the teacher will require for a class of 30:

Coloured labels (enough to label every learner in the sample class)

Counter clicker (not essential but would make sampling easier)

Paper and pencil/pen to record results
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Quiz questions - Answers
1.
A sample of beetles was taken to estimate how many carabid beetles (Carabus
cancellatus) lived in a wood. Spots of paint were placed on the wing cases of the 56
beetles that were caught.
A second sample was taken the next night. A total of 60 carabid beetles were
captured, ten of which had paint mark on their wing cases.
Using the formula:
Total number of carabid beetles =
(a)
Total number in sample 1 x total number in sample 2
Number of marked individuals in sample 2
Estimate the number of beetles there are in the woods.
[2 marks]
56 x 60 = 3360 
3360 ÷ 10 = 168 
Correct answer scores 2 marks, but encourage learners to show their working
out in full.
(b)
List three problems that may lead to the estimate not being accurate.
[3 marks]
Any three from the following:

Marked beetles may affected their attractiveness to predators (more or
less) 

Individuals from the marked sample may have emigrated from the
sampled wood 

The time allowed for marked beetles to fully integrate into the
population may not be long enough 

Sampled individuals may be ‘trap-happy’ 

The paint drop may have been removed 

Any valid point
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2.
A scientist wanted to find out the moth population of a meadow near her house.
She goes out into the meadow with a sweep net and catches 82 moths. She
carefully puts a small spot of white paint onto the thorax of each insect she caught.
Unfortunately, she is not careful enough and kills two during the procedure. She
then releases the marked moths into the meadow. Two days later she returns to the
meadow with a sweep net and catches 67 moths. Of these, 20 have a white spot on
the abdomen.
(a)
Calculate the number of moths in the meadow, explain your working.
[3 marks]
80 x 67 = 5360 
5360 ÷ 20 = 268 
Correct answer scores 2 marks, but encourage students to show their working
out in full.
Record that 82 were captured in first sample but that two were killed in
sampling 
(b)
What is the name of the technique that Jane used to estimate the population
size of the moths?
[1 mark]
Capture-recapture 
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Document updates
v1
v1.1
January 2017
Published on the qualification pages
Consolidated labelling and formatting of activities; removal of
accurate length measurement from apparatus & techniques list.
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Biology PAG 3: Sampling techniques
Combined Science PAG B2: Sampling techniques
Suggested Activity 1: Capture-recapture
Learner Activity
Introduction
The capture-recapture technique is designed to estimate the number of species in a population.
With the capture-recapture technique animals are caught in humane traps. These are marked in
some way (e.g. paint mark, ringing etc.) then released. After a period of time then a second sample
is taken. The number of marked individuals found in the second sample is used to calculate an
estimate of the population size.
In the classic laboratory experiment, woodlice or similar animal are used, either in the wild or in a
large container.
The problem with this approach is that the safe and ethical use of the animals cannot be
guaranteed. In the container the animals can often die, even with the best efforts of the laboratory
staff, as the container cannot exactly match the animals’ normal habitat. Marked individuals can be
easier to spot by predators; so the marking process can be a disadvantage to the animal.
This technique uses your peers as the subjects and can be used to estimate the number of learners
in the centre. This data can then be compared with the known population of the centre and as such
can be used to evaluate the technique and to see how accurate the approach is.
Essential equipment (per group)
Set up:

Coloured labels

A group of people to sample e.g. another class, or in a hospital school – nurses etc.
Version 1.1 –January 2017
9
© OCR 2017
Method
Set up:
This practical is practically simple but it does need a sample to test. The sample must not know why
they are being used as this will skew the result.

Chose a non-GCSE class e.g. a year 7 class.

Inform the ‘sample’ that they are involved in a scientific experiment and that they must not
remove their coloured label until after they leave the site.

Record the number of students that were labelled.

Decide on a suitable time and location to resample. This could be break time/lunchtime/home
time and the tuck shop/dinner queue/centre entrance.

At the chosen time and location record a suitable sample e.g. 100 individuals in total

Identify the number within that sample that are labelled.
Results
Number of individuals in the first marked sample (A):
Number of individuals in the second counted sample (B):
Number of marked individuals in the second sample (C):
Estimated population =
A B
C
=
Actual population:
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Quiz - test your knowledge and understanding
1.
A sample of beetles was taken to estimate how many carabid beetles (Carabus
cancellatus) lived in a wood. Spots of paint were placed on the wing cases of the 56
beetles that were caught.
A second sample was taken the next night. A total of 60 carabid beetles were
captured,ten of which had paint mark on their wing cases.
Using the formula:
Total number of carabid beetles =
Total number in sample 1 x total number in sample 2
Number of marked individuals in sample 2
(a)
Estimate the number of beetles there are in the woods.
[2 marks]
(b)
List three problems that may lead to the estimate not being accurate.
[3 marks]
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2.
A scientist wanted to find out the moth population of a meadow near her house.
She goes out into the meadow with a sweep net and catches 82 moths. She
carefully puts a small spot of white paint onto the thorax of each insect she caught.
Unfortunately, she is not careful enough and kills two during the procedure. She
then releases the marked moths into the meadow. Two days later she returns to the
meadow with her sweep net and catches 67 moths. Of these, 20 have a white spot
on the abdomen.
(a)
Calculate the number of moths in the meadow, explain your working.
[3 marks]
(b)
What is the name of the technique that Jane used to estimate the population
size of the moths?
[1 mark]
DfE Apparatus and Techniques covered
If you are using the OCR Practical Activity Learner Record Sheet (Biology / Combined Science) you
may be able to tick off the following skills:
Biology
6
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Combined Science
6
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