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dependent variable VS measurement proxy: which is which?

These are from edexcel past papers (same specification):

Q1: Scientists have identified five substances (A, B, C, D and E) which might act as inhibitors of perforin. Plan an investigation to test whether these substances inhibit the effect of perforin. Procedure involves using a colorimeter to measure absorbance.

Examiner’s comments: “There was also significant confusion around the dependent variable. It is important that students should know the difference between the true dependent variable and any measurement that may be used as a proxy. In this case the dependent variable under investigation was the activity of perforin, credited via mark point 2, whereas the intensity of color of the solution was the measurement proxy, credited via mark point 7.”

A few years later … another question:

Q2: A student formed the following hypothesis: The greater the concentration of protease the faster the rate of hydrolysis of casein. Plan an investigation to provide evidence to support or reject this hypothesis.

Marks scheme: ‘the dependent variable is the time taken for the solution to become colorless’ or ‘the dependent variable is the time to reach an end point’ or the dependent variable is the absorbance after two minutes’

Why is the dependent variable not activity of protease in Q2, aren’t they expecting the “measurement proxy” as dependent variable?

In the lab book for investigating effect of temperature/pH/concentration on rate of reaction the dependent variable is listed as rate of reaction in absorbance units/s. Which seems to agree with the answer to Q1.

(edited 2 years ago)

Reply 1

englishhopeful98

do they say what the 'measurement proxy' is?

Reply 2

thomas.rhett

Original post by englishhopeful98

do they say what the 'measurement proxy' is?

See the examiners comments for Q1. It's the intensity of color of the solution. They also credited absorbance or transmittance for measurement proxy. see the line from marks scheme for Q1 below.

2. clear statement that the dependent variable is the { activity / eq } of perforin ;

7. description of method of determining dependent variable ; 7. ACCEPT e.g. use of colorimeter to obtain readings for {absorbance / transmittance}

(edited 2 years ago)

Reply 3

englishhopeful98

Original post by thomas.rhett

i meant for the second one

Reply 4

thomas.rhett

Original post by englishhopeful98

i meant for the second one

There is no mention of a measurement proxy in the second marks scheme.

I see the same kind of confusion in the Vitamin C lab. In one past paper (edexcel IAL), the marks scheme expects "Vitamin C concentration" as dependent variable but in the lab book for 2018 specification the dependent variable is given as "volume of juice added to decolorize the DCPIP".

(edited 2 years ago)

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dependent variable VS measurement proxy: which is which?

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