TLDR:
Common stock analysis has undergone significant changes over time, shifting from an emphasis on stable dividends and tangible assets to a focus on future earnings and speculation. The introduction of the new-era theory led to the abandonment of traditional investment principles and a reliance on earning trends, which ultimately proved unreliable in determining a stock's value. The promotion of companies that had already peaked, along with investment trusts' failure to conduct proper research and analysis, made common stock purchases more challenging and risky.