Which is the best chart to prove capability – Histogram or I-MR?

That’s a great question. For capability, I’d go with the Histogram. It’s easy to add the customer requirements to the graph and get a visual on how many times the process fails to meet the specifications. The I-MR Chart is a Control Chart, so it provides a visual on whether there is special cause variation. The Histogram has Upper and Lower Specification Limits and Control Charts have Upper and Lower Control Limits. Traditionally, people don’t put specification limits on Control Charts, but I’ve seen it. I hope that helps!