A Surrogate measure is a measurement taken with the intent to gain insight into a variable that is either impractical to measure directly, or in principle impossible to measure. GDP per capita or PPP are common economic measures that have little meaning in and of themselves but provide insight into the wealth of a nation. In medicine a surrogate outcome measure such as the color of the lips and fingernail beds may be used to inexpensively and non-invasively judge the outcome of a heart surgery.
With a direct and uncomplicated causal relationship surrogate measurements are as good as direct measurements, indeed a measurement taken with a ruler is really a measurement of the photons reflected by a ruler. Difficulty arises when the relationship is not a direct one but rather an illusory one. Physicians discovered in the 18th century that Salacilic acid used to treat fevers lead to gastrointestinal bleeding, and this was used as confirmation for the notion that blood loss lead to humoral balance and good health, and this encouraged blood letting.