Normal Probability Distributions

Excel’s NORM.INV Function

Learning Objectives

Use Excel’s NORM.INV() to calculate x-values related to given areas.

Left Area Given

Bell shaped curve with area to the left of x-value shaded.
Figure 40.1 Area to the left of x-value
  • Use NORM.INV(area, μ, σ) = x
  • To calculate the x-value (or percentile)
  • Corresponding to the area to the left of x

Right Area Given

Bell shaped curve with area above (to the right of) x-value shaded.
Figure 40.2 Area to the right of x-value.
  • Use NORM.INV(1−area, μ, σ) = x
  • To calculate the x-value (or percentile)
  • Corresponding to the area to the right of x

Middle Area Given

Bell shaped curve with middle area shaded between the x-values of x1 and x2. In the middle area is written "Confidence Level". Below the shaded area is written (1−Confidence Level)/2. Above the shaded area is written (1−Confidence Level)/2.
Figure 40.3 Middle area (or confidence level)
  • ‘Middle’ areas can also be called ‘Confidence Levels‘
  • We will use them in later sections also
  • To calculate the lower and upper limits (x1 and x2):
  • We need to calculate the area to left of each x-values
  • The left areas are marked on the graph

Calculating the x-value for a left area (Exercise)

Let us first look at an example where we calculate an [latex]x[/latex]-value when the left area is given.

Example 40.1.1

Problem Setup: Let us revisit the SAT score problem from the previous section. The average SAT score was 1,010 with a standard deviation of 20.

Question: What is the highest score for the bottom 85% of the students?

You try:

You try:

Conclusion: 85% of people score at most 1030.729 on their SATs.

Need Help? Go to the last section for a video that reviews all of the content in this section. You can also download a PowerPoint presentation on Normal Distributions.

Calculating the x-value for a RIGHT area (Exercise)

Let us now look at an example where we calculate an [latex]x[/latex]-value when the right area is given.

Example 40.1.2

Problem Setup: Let us revisit the SAT score problem from the previous section. The average SAT score was 1,010 with a standard deviation of 20.

Question: Above what score do the top 15% of students score?

You try:

You try:

Conclusion: 15% of people score at least 1030.729 on their SATs.

Need Help? Click to reveal the solutions below OR go to the last section for a video explaining all content in this section.

Solutions To This Problem

When we are given the area to the right:

  • We need to take a complement to get the area to the left
  • This is because Excel’s NORM.INV() function works with areas to the left
  • So, for the top 15%, this is the same as the bottom 85%:
Bell curve with right-most 15% of graph highlighted and "x=?" at the start of this area. One thousand and ten is written at the bottom in the middle of the graph.
Figure 40.4 Top 15% of bell curve.
Bell curve with area to left highlighted and 85% written within this area. In the middle below the bell curve 1010 is written. Below, at the end of the highlighted area is written 'x=?'.
Figure 40.5 Lower 85% area in bell curve

To calculate the [latex]x[/latex]-value associated with the above graphs, we use NORM.INV():

\[ x = \text{NORM.INV}(1-0.15,1010,20) = \text{NORM.INV}(0.85, 1010, 20) = 1030.729\]

Calculating the x-values for a MIddle area (Exercise)

Let us finally look at an example where we calculate an [latex]x[/latex]-values when a middle area is given.

Example 40.1.3

Problem Setup: Let us revisit the SAT score problem from the previous section. The average SAT score was 1,010 with a standard deviation of 20.

Question: What is the range of SAT scores for the middle 85% of students?

You try:

You try:

You try:

You try:

You try:

Conclusion: 85% of students score between 981 and 1,038  on their SATs.

Need Help? Click to reveal the solutions below OR go to the last section for a video explaining all content in this section.

Solutions To This Problem

When we are given the middle area:

  • We need to calculate the two [latex]x[/latex]-values separately
  • Input the area to the left of [latex]x_1[/latex] and [latex]x_2[/latex] into NORM.INV
  • The area to the left of [latex]x_1[/latex]: [latex]\frac{100\%-85\%}{2}=7.5\%[/latex]
  • The area to the left of [latex]x_2[/latex]: [latex]\frac{100\%-85\%}{2}+85\%=92.5\%[/latex]
Bell curve with left-most 7.5% of graph highlighted and "x1=?" at the bottom far right of this area. One thousand and ten is written at the bottom in the middle of the graph.
Figure 40.6 Left 7.5% of bell curve selected.
Bell curve with left-most 92.5% of graph highlighted and "x2=?" at the bottom far right of this area. One thousand and ten is written at the bottom in the middle of the graph.
Figure 40.7 Left 92.5% of bell curve selected.

To calculate the [latex]x[/latex]-value associated with the above graphs, we use NORM.INV:

\[ x_1 = \text{NORM.INV}((1-0.85)/2,1010,20) = \text{NORM.INV}(0.075, 1010, 20) = 989.2713\]

\[ x_2 = \text{NORM.INV}((1-0.85)/2+0.85,1010,20) = \text{NORM.INV}(0.925, 1010, 20) = 1038.791\]

Video Explaining ALl topics in this section


Additional Resources:

Key Takeaways (EXERCISE)

Key Takeaways: Excel’s NORM.INV Function

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An Introduction to Business Statistics for Analytics (1st Edition) Copyright © 2024 by Amy Goldlist; Charles Chan; Leslie Major; Michael Johnson is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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