ADX - Average Directional Index
The Average Directional Index was developed by J. Welles Wilder and detailed in his book New Concepts in Technical Trading Systems.
On it's own ADX measures trend strength only. Two additional indicators: the Plus Directional Index (+DI) and the Minus Directional Index (-DI) are used to determine the trend direction.
When +DI is higher than -DI the trend is up and when -DI is higher than +DI the trend is down. An ADX value above 25 could indicate a strong trend and below 20 could indicate no trend.
The calculation is too complex to detail in this short article. It involves an analysis of the High, Low and Close prices for the Period (usually 14) with significant smoothing of results.
Sufficient Data for Accuracy
ADX is one of several indicators that include an element of prior data. As such an ADX based on 50 days of underlying data will be significantly different to a ADX based on 500 days of data. This site will always include enough data to ensure 'accuracy'.