What is weak form efficiency and when would technical analysis be useful or not useful?

Weak form efficiency refers to the idea that past price movements, volume, and earnings data do not affect a stock's price and cannot be used to predict its future direction. In other words, it suggests that all current information is reflected in stock prices, and past information has no relationship with current market prices.

Technical analysis, on the other hand, is a method of evaluating securities based on statistical analysis of their historical price and volume data. The goal of technical analysis is to identify patterns and trends in the market that can be used to make investment decisions.

According to weak form efficiency, technical analysis would not be useful because it is based on past information which is already reflected in stock prices. Instead, investors who believe in weak form efficiency would focus on fundamental analysis, which involves analyzing a company's financials, such as earnings and growth prospects, to make investment decisions.

However, it is worth noting that there are other forms of efficient market hypothesis, such as semi-strong form efficiency and strong form efficiency, which do not make the same claim about past information. These forms believe that past, present, and future information affect stock price movements to varying degrees.