Photographers usually talk about safe and unsafe shutter speeds - those where camera shake is unlikel and those where its likely. The safe shutter speed is the reciprocal of the lens equivalent focal lenght. For example: when shooting at an equiv of 30mm , we should consisder using 1/30 sec as the minimum safe shutter speed. Simple but it's also misleading. Camera shake doesnt just switch on or off at specific shutter speeds. It is always there , but at higher shutter speeds the camera movement doesnt have time to blur the photo significantly. The other factor is luck :). So safe shutter speeds are an estimate , not an exact hard number
With this in mind , it is better to think of them as marginal shutter speeds and not safe shutter speed. This band of marginal shutter speeds still relates directly to focal length. But it gives a better indication of the risk and variability involved. Note that when using a camera or alens with image stabilisation technology , you can take the quoted shutter speed improvement. For example: 2 stops and simply shift the band of marginal shutter speed two stops.