Curious about partition resizing

[squidfaceExtreme]

I've been wondering about something:

Suppose you have an 80GB partition with about 20GB of data on it, and during the installation you want to shrink this partition to 40GB. Can I guarantee that the 20GB of data inside will remain there?

[powerpleb]

You can't guarantee anything, if there is an error or your computer gets switched off or something it could damage the data. You need to backup before resizing partitions.
Having said that, I've often resized partitions and seldom had a problem with it, my data was preserved just fine. Still backup though.

[Partyboi2]

You most likely wont loose you data when shrinking down the partition, but nobody can give you a 100% guarantee, personally I have never lost data when shrinking down partitions.
But I would highly recommend to back up your important stuff before doing any work on partitions in case something does.