The armour is typically supported by ballistic clay but other methods with fabric clamps, rigid frames and anthropomorphic forms can be used with varying results, see our Ballistics Clamps and Fixtures. RTP testing focuses on a specific percentile of interest, typically the velocity at which there is a 50 % probability of perforating the armour (the V50 ballistic limit), or lower percentiles as in the case of the V0 ballistic limit (i.e., VProof). In statistics, sequential testing involves a sample size that is not fixed in advance. Instead, data are evaluated as they are collected, and further sampling is stopped in accordance with a pre-defined stopping rule as soon as significant results are observed. Thus, a conclusion may sometimes be reached at a much earlier stage than would be possible with more classical hypothesis testing or estimation, at consequently lower financial cost. Resistance-to-perforation testing measures the ability of armour to withstand perforation by kinetic energy projectiles (i.e., bullets) or fragment simulating projectiles (FSP). This property is determined by firing projectiles at the armour target and varying the velocity from round to round to explore target performance.