The audibility measurement is based on regular ratings of the best BBC World Service short-wave frequency in any given area.
A weighted average score for each language and region is calculated and expressed as a percentage: 100% represents outstanding, while 70% is good-quality audibility.
In practice, variations of up to 5% will not have a significant effect and the impact of even a 10% change will only be moderate.
Variations in short-wave propagation are inevitable because of changing ionospheric conditions. Unusual solar activity throughout 2000/01 may have exaggerated this effect.
