# Inaccurate Area under normal curve

I am using geogebra to provide interactive graph of the standard normal distribution for a WeBWorK question.

The area under the curve is not being calculated with as much accuracy as I need.

P(Z<-1.91) is 0.0281 but setting the sliding point to -1.91 on the graph shows an area of 0.0274.

You can find the graph in question at https://www.geogebra.org/m/a9z9GKKB.

I have used Normal(meanPop, stdDevPop, x) to graph the function with meanPop=0 and stdDevPop=1.

Any ideas?

hello

change xminVal by -100, i.e.

saludos

1

use the probability calculator for this question. You can find the tool in the toolbar of the Spreadsheet.

It gives the correct result

chris

1

Unfortunately, the probability calculator does not export to WeBWorK, though this would be my preference.

1

hello

change xminVal by -100, i.e.

saludos

1

Yep. I tried to do with fewer variables by using the xMinVal and xMaxVal that can be reset in WeBWorK for the area limits as well. This is exactly the problem. Though now that I think about it I can hard code the outside tail limits to -100 and +100 since they'll never need to be altered.

Thank you kindly, I am sure this will take care of it.

1

If you want it even more accurate, you can define the pdf and use that:

1. F(x) = -1 / 2 erf(-x sqrt(2) / 2 )

2

"the probability calculator does not export to WeBWorK"

you can show the probability calculator in the main window by clicking on the icon in the upperright corner of the prob.calc. Then it behaves as any GG file with the same possibilities of exportation.

chris