Home / Statistical Tools / Analysis Tools / Cpkhistogram / Options
Options¶
The Cpk/Histogram dialog has two option tabs: Capability Options for statistical settings and Chart Options for display and distribution fitting settings.
Capability Options¶
Process Capability Preset¶
Controls which capability indices are displayed and which standard deviation component each uses.
| Preset | Description |
|---|---|
| AIAG | Industry standard. Cpk uses within-subgroup StDev, Ppk uses overall StDev. Both Cpk and Ppk are displayed. This is the default. |
| Boeing D1-9000 | Cpk uses overall StDev. No separate Ppk is displayed. |
| Custom | Full control — choose which indices to show (Cpk, Ppk, or both) and which StDev component each uses (within or overall). |
Transformations¶
Apply a data transformation before calculating capability indices. Useful when your data is not normally distributed.
| Option | Description |
|---|---|
| None | Use raw data as-is (default) |
| Johnson | Applies an automatic Johnson transformation (SL, SB, or SU family selected based on data shape) |
| Box-Cox | Applies a power transformation. Lambda is calculated automatically for optimal normality, or you can enter a custom lambda value between -5 and 5. Requires positive data values. |
Override Population Parameters¶
Optionally enter known population values instead of estimating them from the data:
- Population Mean — Override the calculated mean
- Population Standard Deviation — Override the overall standard deviation
- Population StDev Within Subgroups — Override the within-subgroup standard deviation
Leave blank to use the values estimated from your data.
Estimate StDev Within Subgroups¶
Controls how the within-subgroup standard deviation is estimated. The available methods depend on your subgroup size.
For subgroup size = 1 (individual observations):
| Method | Description |
|---|---|
| Average Moving Range | Uses the average of consecutive moving ranges to estimate variation (default) |
| Median Moving Range | Uses the median of consecutive moving ranges, which is more robust to outliers |
Moving Run Length controls how many consecutive observations are used for each moving range calculation. Default is 2.
For subgroup size > 1:
| Method | Description |
|---|---|
| Pooled StDev | Pools the standard deviations from all subgroups (default) |
| R-bar | Uses the average range within subgroups |
| S-bar | Uses the average standard deviation within subgroups |
Unbiasing Constants¶
When checked, applies correction factors (d2, d3, c4) to the standard deviation estimates. These constants account for bias in small-sample estimates and are recommended for small subgroup sizes.
Confidence Intervals¶
- Show Confidence Interval — Toggle display of confidence intervals for capability indices
- Mode — Two-Sided (default), Lower Bound only, or Upper Bound only
- Confidence Level — Enter as a decimal between 0 and 1 (default 0.95 for a 95% confidence interval)
Confidence intervals are calculated for Cp, Cpk, Pp, and Ppk.
Goodness of Fit¶
Tests whether your data follows a normal distribution. Select one test or None.
| Test | Description |
|---|---|
| Anderson-Darling | Emphasizes the tails of the distribution. Reports A² statistic and p-value. Default. |
| Shapiro-Wilk | Best for small to moderate sample sizes. Reports W statistic and p-value. |
| Kolmogorov-Smirnov | Uses Lilliefors correction. Reports D statistic and p-value. |
| None | Skip normality testing. |
Chart Options¶
Display Options¶
- Display Histogram Bars — When checked, shows the histogram bars representing the frequency distribution. Enabled by default.
- Display PDF Curve — When checked, overlays probability density function curves on the histogram. Two curves are shown: one using within-subgroup StDev and one using overall StDev. Enabled by default.
Distribution Fitting¶
- Fit Distribution — When checked, fits a statistical distribution to your data. The fitted curve replaces the normal curve overlay.
Available distribution types:
| Distribution | Description |
|---|---|
| Normal | Standard bell curve (default) |
| Lognormal | For right-skewed positive data |
| 3-Parameter Lognormal | Lognormal with location parameter |
| Exponential | For time-to-event data |
| 2-Parameter Exponential | Exponential with location parameter |
| Weibull | Flexible shape for reliability data |
| 3-Parameter Weibull | Weibull with location parameter |
| Logistic | Similar to normal but heavier tails |
| Log-Logistic | For right-skewed data with heavy tails |
| 3-Parameter Log-Logistic | Log-Logistic with location parameter |
| Smallest Extreme Value | Gumbel distribution for minimum values |
Quantum XL tests each distribution against your data and shows fit statistics (p-value, AICc, BIC) to help you choose the best fit.
Bin Configuration¶
Controls how histogram bins are created. Settings are per data column.
- Number of Bins — Auto (Quantum XL calculates optimal count) or manual entry. Mutually exclusive with Bin Width.
- Bin Width — Auto or manual entry. Mutually exclusive with Number of Bins. Changing one recalculates the other.
- Left Point — The minimum X value for the histogram. Auto or manual entry.
Non-Normal Capability Method¶
Controls how capability indices are calculated when a non-normal distribution is fitted. Only enabled when a non-normal distribution is selected in Distribution Fitting.
| Method | Description |
|---|---|
| Percentile | Uses the fitted distribution's percentiles to calculate capability (default) |
| Z-Score | Uses z-score transformation based on the fitted distribution |
Plot Order¶
Controls the draw order (Z-order) of overlaid data series when multiple data columns are combined into a single chart. The first series in the list is drawn first (back layer), and subsequent series are drawn on top.
Use the Up and Down buttons to reorder the series. This is useful when overlaid histogram bars obscure each other — move the series with fewer bars to the front so all data is visible.
Only available when multiple data columns are selected and combined into a single chart.