Qualitative research uses the subjective measure of observations which is not based on structured and validated data-collection. ... Qualitative research is not double-blind, and allows bias into the research: this alone invalidates an entire study and makes it worthless.

Is a double blind study qualitative or quantitative?

Quantitative researchers TEND to:

try to tightly control the variable(s) in question to see how the others are influenced. Do experiments, have control groups, use blind or double-blind studies; use measures or instruments.

Are RCTs always quantitative?

Randomized controlled trials are quantitative, comparative, controlled experiments in which treatment effect sizes may be determined with less bias than observational trials.