Companies in the process and energy industries need to know not only how safe they have been, but also how safe they are likely to be in the future. This is where the concepts of leading and lagging indicators come in.
Lagging indicators analyze past events. They are reactive — they show where systems failed. Leading indicators measure conditions, behaviors, or processes that help predict and prevent incidents before they occur. Lagging indicators show consequences and trends; leading indicators provide actionable early warnings.

Lagging indicators identify barriers that failed. Leading indicators identify the barriers that are weakening.

A detailed post, that includes examples, is provided at the post Leading and Lagging Indicators in Process Safety Management.