x and R charts are the control charts used for continuous data, e.g., the weigth or lenght of a product, the time to produce an item or reply a phone call...

x charts 

x charts show changes in the mean tendence of the process. 

Upper control limit (UCL)	Central line	Lower control limit (LCL)

Where:

x̿ is the mean of the samples;

 is the standard deviation of the process (σ) divided by the square root of the elements per sample (n);

and typical values for z are 2 (more restrictive) or 3 (less restrictive).

R charts 

R charts show changes in the dispersion of the process.

Upper control limit (UCL)	Central line	Lower control limit (LCL)

Where:

R is the average range of the samples;

and D₄ and D₃ are taken from the 3-sigma table.

Building x and R charts 

1. Take 20-25 samples of 4-5 pieces/products each one. Calculate mean and range for each sample.
2. Calculate global mean and range ( and ).
3. Fix desired limits (generally 3σ) and calculate actual control limits
4. Draw samples in the chart.
5. Analyze the results and research for the possible causes of variation

Explanatory video