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Chance and Data dot plots

For the dot plot question (bivariate), when it asks you to find the average, do you use the rise/run method? and if so how do you do this.

e.g if the question said: State the relationship that exists between the age of cars and the distance travelled AND find the average change in distance travelled by a car each year as the age increases.

I can’t add the picture, but could you just use any numbers because I don’t know how to use the rise/run method

Thanks

Hi @char1otte

This would be based on your line of best fit (which often you need to put on yourself). You want to put a line through going in the direction of the dots, having half on each side. Then find some key points that lie on this line with nice values. The “rise/run” isn’t too complicated, you simply work out how far up and how far across then simplify.

For example say your graph goes through (5,12) and also (8, 18)
The “rise” would be 18-12=6
The “run” would be 8-5=3
So the gradient would be 6/3 which is simplified to 1/2. This means that for every one square we go across we are expecting it to rise by 1/2.

Hope this clarifies things. If there are any specific NCEA questions you want help with please ask and we can help :slight_smile:

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Thanks for your reply

Is the gradient the same as the rise/run ? or is that different? and also, do the dots for the rise/run have to be on the line of best fit?

Gradient is calculated as rise/run. You are not expected to give a very precise answer. Your best shot would be to draw a trendline (the one which fits your data the best) and estimate the gradient of this line. You can say for example, that each year the car drives approximately 10,000km, increasing the distance it has traveled by an average 10,000km a year.

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