I am wondering about which sentence is correct?
It is therefore measuring the volatility of an investors asset.
It is therefore measuring the volatility of an investor's asset.
I agree with Imjohns3; the correct answer is number two.
The use of an apostrophe to indicate possession or ownership of, or belonging to, is relatively straightforward. Here are a few rules and examples:
1.) With a singular noun (even if it ends in S), simply add an apostrophe after the singular noun, and add an S.
For example, the cat's toy, the bus's windows, the desk's color, the banana's texture, the fish's taste, the gas's fumes, Sally's chair, Bob's wallet, Willis's teeth, etc.
In other words, the toy belongs to the cat, the window belongs to (is a part of) the bus, the color "belongs to" (is a part of) the desk, Willis's teeth are the teeth that belong to a guy named Willis, etc.
2.) With a plural noun, whether or not it ends in S, simply add an apostrophe after the plural word and then add an additional S
For example, the cats' toy, the cats' toys, the buses' windows, the apples' weight, the people's opinion (people is plural), the men's room, the women's clothing, the Jones's house (there are two or more people with the name of Jones who possess the house), etc.
In other words, the one toy belongs to (is owned or shared by) two or more cats; two or more toys belong to (are owned or shared by) two or more cats; the windows belong to (are part of) two or more buses; the weight belongs to two or more apples; the room is owned by more than one man (a "men's restroom." on the other hand, is where men--plural--are free to go), etc.
3.) When singular or plural nouns end in S or SS, if by adding an additional S the word's pronunciation becomes difficult, put only an apostrophe after the word.
For example, Moses' leadership in Israel, Jesus' parables, the glasses' weight, the Joneses' family reunion, the breakfasts' combined cost, etc.