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MVPstats — Statistic Descriptions

The following a brief discussion of statistics found in the Explore routines. Confidence intervals are also available for the Mean, Standard Deviation, and Variance.

Included on this page are:

Measures of Location Measures of Position Measures of Variability Measures of Shape Distribution Testing Statistics Natural Tolerance Statistics Capability and Performance Statistics

Measures of Location

Mean

The mean is also known as the average, calculated as follows.

XbarCalc

Median

50% of the data fall at or above the median and 50% of the data fall at or below the median. The median is found in the (n+1)/2 position in the data, when the data is sorted from low to high.

Interpolated Median

When data are grouped an interpolated median may be used. This is also useful for low resolution data as implemented in MVPstats, where several values fall at the median. The following formula is used.

Median-Interpolated

Where

Measures of Position

Low

Lowest value in the data set.

Xlow

High

Highest value in the data set.

Xhigh

Q1

Q1 is the first quartile. It is the value at or below which 25% of the data fall. Q1 is using the (n+1)/4 position in a data set sorted from low to high and interpolating.

Q3

Q3 is the third quartile. It is the value at or above which 25% of the data fall. Q3 is found in the 3(n+1)/4 position in a data set sorted from low to high and interpolating.

Percentiles

The Pth percentile is the value that P% of the values fall at or below, and (100-P%) fall above it.

Percentiles are found by sorting the data from low to high, and interpolating when necessary. The Pth percentile may be found in the P(n+1)/100th position, where P is expressed as a percent. If the percentile position is not an integer the percentile falls between two values. In this case, the fraction generated in the position calculation yields the fraction of the difference between the two values. If P is less than 100/(n+1) or greater than 100 n /(n+1), P is undefined.

Measures of Variability

Range

The Highest minus the lowest value.

R = Xhigh - Xlow

Std Dev - Standard Deviation

The standard deviation is a measures of variability. It is the square-root of the average squared distance each value falls from the mean.

StdDevCalc-eqn

Variance

The variance is simply the standard deviation squared.

S2

Coefficient of Variation

The coefficient of variation is calculated as follows.

CV-Calc

Std(MR) - Std Dev using Average MR

The is another estimate of the standard deviation, using the average Moving Range (the average of the moving absolute difference of sequential values).

MR over d2

where

Std(MMR) - Std Dev using Median MR

This is another method to estimate the standard deviation, similar to Std(MR), except the median of the moving ranges is used.

MRTildaOverD2-eqn

where

IR - Interquartile Range

This is the range of the middle 50% of the data.

IR = Q3 - Q1

PSD - Psuedo Std Dev using IR

This is another method to estimate standard using the Interquartile Range divided by a constant, assuming a normal distribution.

PSD = (Q3 - Q1) / 1.3489795

SIR - Semi-Interquartile Range

This is simply 1/2 the Interquartile range.

SIR = IR / 2

MD(Med) - Mean Abs Dev about Median

This measure is related to the standard deviation, except it measures absolute deviations around the median. The MD(Med) is calculated as follows.

MADCalc

MD(Avg) - Mean Abs Dev about Average

This measure is related to the standard deviation, except it measures absolute deviations around the mean or average. The MD(Avg) is calculated as follows.

MAD(Avg)Calc

Measures of Shape

Skewness (See Skewness & Kurtosis)

SkewnessCalc

Where

MomentsEqn

Kurtosis (See Skewness & Kurtosis)

KurtosisEqn

Distribution Testing Statistics

A-D - Anderson-Darling Test for Normality Test Statistic

L-M - Lin-Mudholkar Test for Normality Test Statistic

W(E) - Shapiro-Wilk Test for Exponentiality Test Statistic

Poisson Test Statistic - See Poisson Distribution Test.

Natural Tolerance Statistics

-4s, -3s, -2s, -1s, +1s, +2s, +3s, +4s

These are the values corresponding to given standard deviations from the mean, as follows.

As = Mean + As

where “A” is a positive or negative number

4s, 6s, 8s

These may be used for natural tolerances, assuming normality.

NT = As

n below -3s, n above +3s

This is the count of observations below 3 standard deviations, or above 3 standard deviations.

Capability and Performance Statistics

LSL, Target, USL

These are the Entered Specifications.

n Below Spec, n Above Spec, n Out Spec, % Below Spec, % Above Spec, % Out Spec

These are the count or percent of observations below, above, or total outside of specifications.

Est %Below Spec, Est %Above Spec, Est %Out Spec

These are estimated proportions outside of specifications, assuming normality.

Cp, Cpk, Cpm

These are capability measures, estimated using sample statistics.

Pp, Ppk, Ppm

These are performance measures, which are calculated the same as capability measures using sample statistics.

See Ppm, Ppk, Pp, %Off-Target

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