Quantitative data involves numbers. Qualitative data involves descriptors.
“Great. So what?” was my response for the longest time.
Then one day, Laura Klein finally got me to see the light:
The power of quantitative and qualitative data lies not so much in what they are, but in how they work together.
Quantitative data identifies what’s happening.
Qualitative data explains why it’s happening.
In others words, you collect quantitative data to identify a problem within your user experience (e.g. X number of users abandon their cart just before they enter a credit card number). And then you gather qualitative data to explain why the problem exists (e.g. they’re worried that someone will steal their credit card number).
Notice the “gather qualitative data” part, as in don’t try to explain what’s happening on your own. Let your users tell you ;)