A purchaser presumes that they are buying shares that are worth "X" if they drop in value by 20% or some specify percentage then - that's bad - and the purchaser would not be interested anymore. A material adverse change clause can be quite detailed - and specify what "material" means and what "adverse" means . What is "material" can be different from business to business and can be measured by the financial statements ie ongoing revenues, or nature of the business, or composition of the team (so qualitative factors). What is adverse would be some negative effect.