# How to make a fair comparison between average of two groups?

Here's the scenario.

When users add an item in an auction listing, they can opt for an "upgrade" where more people are likely to bid.

How can I get how times more, that items in the auction listing with the "upgrade" get bids than those without?

Here is the current data. This is an example only.

1. There are more items with no upgrade.

• Count of items w/o upgrade: 1500
• Count of items w/ upgrade: 300
2. Total no. of all bids

• Bids on items w/o upgrade: 30000
• Bids on items w/ upgrade: 10500

I could simply get the number of bids per item and compare between those with and w/o upgrades

• Bids per item w/o uprade: 30000 / 1500 = 20
• Bids per item w/ upgrade: 10500 / 300 = 35

So it's 75% (35/20 - 1) more bids if an item has upgrade.

So my question is, how to make a more fair comparison how much bids an item gets with upgrade vs without? Since according to data, there are much more items with no upgrades and these are usually low-valued ones (say avg of 100 USD versus avg 500 USD for those w/ upgrades). Should I also put dollar value in the formula here?