Static coefficient of friction (SCOF) is described as the amount of required pressure between two surfaces to prevent slippage.
Coefficient of friction (COF) is a mathematical term used to describe the effect of dragging one substance (shoe sole material) over another (flooring surface).
This coefficient is a measurement of the relative ability of various surfaces to resist the sliding or slipping of the selected material. 
The ADA requires a SCOF of .5 for all public access areas, while OSHA requires a SCOF of .6 on walkways and .8 on ramps.
Unglazed tiles have a greater slip-resistance than glazed tiles and are commonly recommended for areas subjected to high water spillage. Irregular or textured surfaces can be confused for slip-resistant surfaces. Because a glazed tile is rough or not glossy, doesn’t necessarily mean it is slip-resistant. Even many irregular or textured unglazed tiles can become slippery when wet, allowing surface hydroplaning to occur.
Many glazed and unglazed tiles can feature abrasive grit on their surface, increasing their slip resistance substantially. These tiles are commonly installed in public areas with direct access to the outdoors. Corundum grit surfaces can introduce an element, when traffic acts upon the surface, which will add to the floors deterioration and can be too slip-resistant, when excessive.
Please Note: Any tile or other hard surface flooring can become slippery when wet or improperly maintained. Slip resistance varies with the many types of footwear, soiling, and cleaning regiment.
Avoid trying to raise the COF by using coatings as these are susceptible to peeling, blistering, discoloration and marking. This may create a floor surface that is difficult to maintain, as well as potentially unsatisfactory slip-resistance.