Can subjective data be studied quantatively?

The history of psychology is filled with examples of studying subjective experiences quantitatively. For example: light frequencies were varied to discover the minimal differences in frequency subjects required to reliably categorize color; physiological arousal, as measured by instrumentation, has been used to gauge emotional preferences; reaction times have been recorded as a measure of the difficulty of a task; psychological tests measure all kinds of individual differences in responses to the same stimuli. All of these examples involve using measurements expressed in numbers (e.g., light frequencies, electrical resistance, time passing, and percentages of populations that respond in given ways), and are therefore quantitative measurements.