LIFE SCIENCES:CONSOLIDATION TASK 9

TOPIC: HYPOTHESIS TESTING

Concepts to be understood and to be able to apply in unfamiliar context

Hypothesis testing

Teachers in Grades 10, 11 and 12 must ensure that their SBA tasks include an investigation in which skill 7 is emphasised. Special attention should be paid to the following:

the stating of a hypothesis;

dependent and independent variables;

precautionary measures;

reliability;

validity, and

interpreting results.

What is a hypothesis?

A hypothesis is an attempt to explain some event or observation using whatever information is currently available.

How to state a hypothesis

A hypothesis must:

(a)havetwo variables (dependent and independent variables);

(b)state the relationship between the two variables;

(c)betestable; and

(d)state the independent variable first (cause) and then the dependent variable

(effect).

An example of a hypothesis question is as follows:

The peppered-moth, Bistonbetularia, has two phenotypes for body colour, dark (blackish) and pale (whitish). The trunks of the trees on which the moths rest are black in polluted environments compared to the white trunks of trees in unpolluted environments. In both unpolluted and polluted environments, birds are the predators of the moths. An investigation was carried out to determine the number of dark and pale peppered moths present in polluted and unpolluted environments, using a sampling technique.

Formulate a hypothesis for the above investigation.

More √/fewer dark peppered moths √/ pale peppered moths survive in the polluted √/unpolluted environment

OR

No difference √ in the number of dark √ / pale peppered moths that survive in both √ environments

Learners have to be able to differentiate between validity and reliability in scientific investigations, because the principles of validity and reliability are fundamental cornerstones of the scientific method.

What is reliability?

The idea behind reliability is that any significant results of an investigation must be more than a once-off finding and be repeatable.

Other researchers must be able to perform exactly the same experiment, under the same conditions, and generate the same results.

This would reinforce the findings of the experiment and ensure that the wider

scientific communityaccepts the hypothesis.

For the questions which required learners to state how the reliability of the investigation could havebeen improved, the following answers apply:

Repeat the investigation OR Increase the sample size.

What is validity?

Validity questions show how the experiment/investigation was carried out. It is important to be sure thatall the factors/variables have been controlled/fixed except the variable/factor being tested.

The samples must be chosen randomly.

The design for the investigation must be appropriate.

Validity therefore speaks to whether the scientific research method was used with the appropriate degree of care and diligence.

In questions which require learners to suggest some factors that might have decreased the validity of an investigation, the answers should centre on the criticism of the scientific process; for example, some factors/variables that were not fixed/controlled when carrying out the investigation.

Examples

1There is variation in the wing length of fruit flies (Drosophila melanogaster).

Some have long wings and can fly while others have short wings and cannot

fly.

An investigation was conducted to determine which flies would survive under

certain conditions.

The following steps were carried out:

  1. Five flies with short wings and five flies with long wings were placed in

a flask.

  1. Food was placed at the bottom of the flask.
  2. The lid of the flask allowed airflow.
  3. Sticky paper was suspended from the top of the flask. Flies that got stuck

to the paper died.

  1. The apparatus was left for 24 hours.

The results of the investigation are shown in the diagram below.

1.1Formulate a hypothesis for the investigation.(3)

1.2Explain why it is necessary to have openings for airflow.(3)

1.3State TWO ways in which the reliability of the investigation could

be improved.(3)

1.4Other than the opening for airflow, explain FOUR other precautions

that should be taken in this investigation.(8)

2A farmer conducted an investigation to determine which type of fertiliser

would increase the yield of her wheat crop.

•She divided her farm into three 1 hectare plots and treated them as

follows:

•She planted the same type of crop, namely wheat, during November

•eachyear for five years.

•She used water from a river which flows through the farm to irrigate her

crop.

•She recorded the yield per plot for each year. The yield was measured

•bycalculating the number of kilograms of wheat produced per hectare.

2.1Identify the dependent variable in this investigation. (1)

2.2Explain the purpose of including hectare A in this investigation. (2)

2.3State ONE way in which the farmer could have increased thereliability

of her results.(1)

2.4If this investigation was carried out for more than five years, listTHREE

negative effects of planting the same type of crop overmany years on the

same plot of land.(3)

2.5Explain how the excessive use of fertilisers can affect biodiversity ifit is

washed into the river.(8)

MEMORANDUM

QUESTION 1

1.1More √/ fewer long winged √/ short winged flies will survive √/die

OR

Equal numbers √ of both types of flies√ will survive √/ die (3)

1.2CO2 to move out and O2 to move in √/ ventilation

To allow breathing √ / respiration

So that flies do not die √ / suffocate(3)

1.3repeat the investigation √

Increase the number of flies √

Using many flasks √ / replications any(2)

Mark first TWO only

1.4Ensure that the flies do not come into contact with the sticky paper √

When placing them in the flask so that their death is not caused by the

investigator√

Ensure efficient food supply √ for the period of the investigation so that

the death is not due to hunger √

The openings for airflow should be small √ enough so that

The flies cannot escape √ / or others enter

Maintain optimum environmental conditions √to allow the flies to

survive √ /behave normally4 x 2(8)

QUESTION 2

2.1Number of kilograms of wheat per hectare √ / yield(1)

2.2To compare √ theyield obtained when using two types of fertiliser

with the yield of the hectare with no fertiliser √

OR

It acts as a control √ to ensure that the results obtain are due to the

addition of fertilisers √ and not any other factor(2)

2.3she could have increased the sample size √ /number of plots /

Number of plants for each type of fertiliser used

Repeat the investigation √(1)

Mark first ONE only

2.4Depletes nutrients in the soil √

Leads to decrease in yield √

Increase pests √

Leads to soil erosion √

Decreases biodiversity √Any 3(3)

Mark first THREE only

2.5increases the nutrient content √ of the surrounding river /

eutrophication occurs / water becomes polluted

this causes an increase in algae growth √ /bloom

the algae block out light √

reducing photosynthesis √

plants and animals depending on them die √

increasing decomposition √

leading to a depletion of oxygen√

and reducing the biodiversity√in the river(8)