Friday, July 22, 2011

Measures of Variance

Download a PDF of the Assignment Here


So far we’ve been talking about measures of Central Tendency: mean, median, and mode. Central tendency will provide a quick glimpse of the data in terms of where the middle number is, which number occurs the most, and what is the average of all the numbers. Central tendency provides great information that can be used for relations and comparisons. The average can be used to tell how well did one student do compared to the class average? Mode can be used to tell you which test number most of the students missed? Yet, it doesn’t provide a complete picture of the data, for example the mean and median can be the same. Does that mean the data set is the same? No.


Once we have a data set, we can analyze it in many different ways. We want to now look at Measures of Variance. Variance will tell you how spread out your data set is in relation to the mean. Do you have a standard distribution? Normal curve? What? Why is that important?

What is Standard Deviation?
Let’s start with what is a standard deviation. From Wikipedia, “It shows how much variation or "dispersion" there is from the average (mean, or expected value). A low standard deviation indicates that the data points tend to be very close to the mean, whereas high standard deviation indicates that the data are spread out over a large range of values.” http://en.wikipedia.org/wiki/Standard_deviation


A very high SD usually means that your data are too variable and therefore are unreliable.

Best part of the assignment, we can use the Standard Deviation function in Excel to do all the calculations for us. Review the screencast on how to calcualte StDev: http://www.screencast.com/t/B4soGICZaLXz












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Here’s some more information and background for Standard Deviation before the class assignment:



Assignment: Download and use the data set provided here: ISAT 4th Grade, to:



  1. Calculate the Measures of Central Tendency for each content area

  2. Calculate the Highest number and the Lowest number for each content area

  3. Calculate the Standard Deviation for each content area

  4. Post your spreadsheet to the Discussion Board and answer the following question:

    What is the Standard Deviation of Earth Sciences and what does that score represent?

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