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July 20, 2019, 11:29:10 pm

Author Topic: Variables, Uncertainty and Error  (Read 78 times)

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Bri MT

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Variables, Uncertainty and Error
« on: June 21, 2019, 03:37:59 pm »
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For both year 11 and 12, science subjects require you to understand and discuss variables and uncertainties.


Variables

Dependent variable (DV):
This is the variable which is being changed as a result of other variable/s in the experiment. (its value is dependent on the other variables)
-This variable belongs on the y-axis of a graph
-You want the changes to only be caused by the independent variable

Independent variable (IV): This is the variable which you manipulate to determine the impact it has on the dependent variable. (its value is independent of the values of the other variables)
-this value belongs on the x-axis of the graph
-You want this to be the only variable impacting the dependent variable/s

Extraneous variable (EV): This is a variable which might have an impact on the dependent variable
-you do not want these variables around
-get rid of them if you can
-keep them constant if you can (ie. make them  controlled variables  )
-you DO want to talk about the potential impacts of these
-quantify their impact if you can
-if you canít control them, or they werenít controlled in the experiment you looked at definitely still talk about them
-the reason we don't like them is because it makes it hard for people to know if things happened because of the IV or because of EVs, which then makes it very difficult to draw conclusions about the relationship between the IV and DV


Uncertainty

Uncertainty is also sometimes known as error but you should be careful with using the term "error" as 'making a mistake' and 'having uncertainty' are very different things. Uncertainty is often show on graphs as bars which extend below and above the y-value, but uncertainty can also apply to the x-value - this will depend on your experiment and the nature of your data.

Whatís the uncertainty of my raw data?
- things like analytical scales should tell you what the uncertainty is somewhere on the device
- you can look up the uncertainty of particular types of glassware online
- For an analog instrument, you can cite uncertainty as +/- half the distance between two adjacent markings  (unless uncertainty is provided by manufacturer in which case you should use that)

Propagating uncertainty:
- this refers to finding the uncertainty of your answe when you put a data with uncertainities into an equation
-if adding or subtracting values, add the uncertainties together to find the uncertainty of the new value
-if multiplying or dividing values, add the percentage/relative uncertainties together to find your new values

Basic types of uncertainty:
Random: The measured values are different to the actual values in a different way each time (eg. one time measure the value as 0.5 mm more than reality, the next you measure it as 1.0mm less than reality)
-usually due to humans
-account for these by repeating multiple times and averaging, that way the shifts to either side should balance each other out

Systemic: The measured values are different to the actual values in the same way each time (eg. thermometer always measures 0.5 K higher than it should)
-usually due to instruments



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