MSA 1 -- Measurement System Analysis Method 1

The MSA Method 1 (also known as Procedure 1 or Cg/Cgk Study) serves to assess the repeatability of a measurement system based on a single part. It is the simplest and fastest method to check the fundamental capability of a measuring instrument.

Overview

Purpose and Application

The MSA 1 answers the question: Is my measuring instrument fundamentally capable of resolving the required tolerance?

In this process, a single part (reference part, standard, or master piece) is measured repeatedly -- typically 25 or 50 times. From the variation of these repeat measurements, the capability indices Cg and Cgk are calculated.

When is MSA 1 used?

• During initial qualification of a new measuring instrument
• During periodic verification of existing measuring instruments
• For preliminary testing before a more extensive MSA 2 is conducted
• After repair or calibration of a measuring instrument

Info: MSA 1 evaluates exclusively the repeatability and the systematic deviation (Bias) of the measuring instrument. Influences from different operators or parts are not considered -- MSA 2 is intended for that purpose.

Typical Procedure

1. Provide reference part with known reference value
2. Enter tolerance limits and reference value in my8data
3. Measure reference part multiple times (minimum 25 measurements recommended)
4. Enter measured values in my8data
5. Perform calculation and evaluate results

Input

Reference Value and Tolerance

Before you enter the measured values, the following parameters must be established:

Important: The reference value must have been determined using a higher-level, more accurate measuring instrument (e.g., coordinate measuring machine, calibrated standard). The accuracy of the reference value should be at least 10 times better than that of the measuring instrument being tested.

Enter Measured Values

Enter the repeat measurements in the input table:

1. Click in the first cell of the measured value column.
2. Enter the first measured value and confirm with Enter or Tab.
3. Continue until all measured values have been entered.

Alternatively, you can paste measured values from Excel using Ctrl+C / Ctrl+V (see chapter Enter Data).

Tip: For a meaningful analysis, at least 25 measurements should be performed. Many standards and guidelines recommend 50 measurements.

Additional Input Fields

Depending on the configuration, additional fields may be available:

• Characteristic designation -- Name of the measured characteristic (e.g., "Bore diameter 3")
• Measuring instrument -- Designation of the measuring instrument used
• Operator -- Name of the operator
• Date -- Date of measurement

Indices

After calculation, the following statistical indices are determined and displayed:

Cg -- Repeatability Capability Index

The Cg value (Capability of the Gauge) describes the repeatability of the measuring instrument in relation to the tolerance. It considers only the variation of the measured values, not the systematic deviation.

Formula:

Cg = (k * T) / (s * sg)

Where:
- k = Tolerance factor (Standard: 0.2, i.e., 20% of tolerance)
- T = Tolerance (USL - LSL)
- s = Spreading factor (Standard: 6 for 99.73% range)
- sg = Standard deviation of measured values

Cgk -- Repeatability Capability Index with Bias

The Cgk value additionally considers the systematic deviation (Bias) of the measuring instrument from the reference value. It is always less than or equal to the Cg value.

Formula:

Cgk = (k * T - |xm - x_quer|) / (s * sg)

Where:
- xm = Reference value
- x_quer = Mean of measured values

%GRR (Portion of Tolerance)

The %GRR value indicates the percentage portion of the measuring instrument variation in relation to the total tolerance.

Evaluation Criteria

Tip: If the Cg value is good but the Cgk value is poor, a systematic deviation (Bias) exists. In this case, the measuring instrument should be recalibrated.

Info: The threshold values can vary depending on the industry standard. In the automotive industry, threshold values of 1.33 according to VDA or AIAG are frequently used.

Diagrams

MSA 1 in my8data generates two central diagrams for visual assessment of the measurement system.

Run Chart

The run chart displays the individual measured values in the order they were recorded:

• X-axis: Measurement No. (1, 2, 3, ...)
• Y-axis: Measured value
• Red horizontal line: Reference value (xm)
• Blue horizontal lines: Upper and lower tolerance limits (based on tolerance portion)
• Green points: Individual measured values

The run chart helps you recognize the following patterns:

• Trend: Systematic drift of the measuring instrument over time (e.g., due to heating).
• Jumps: Sudden changes that indicate external influences.
• Uniform variation: Measured values should scatter randomly around the reference value.

Warning: If you observe a clear trend in the run chart, the prerequisite of stable measurement is violated. Investigate possible causes (e.g., thermal influences, wear) before interpreting the results.

Histogram

The histogram shows the distribution of measured values:

• X-axis: Measured value ranges (classes)
• Y-axis: Frequency
• Red vertical line: Reference value
• Blue vertical lines: Tolerance limits (based on tolerance portion)
• Bell-shaped curve: Normal distribution fit

The histogram enables you to:

• Assess the symmetry of the distribution (ideally bell-shaped and symmetric around the reference value).
• Visually estimate the width of the variation in relation to the tolerance.
• Recognize outliers or multiple peaks that could indicate measurement problems.

Tip: A histogram with a clear shift from the reference value indicates a systematic deviation (Bias). This is also reflected in a lower Cgk value.