Well, dur. Being a great game aside, multi-platform games typically do better thanks to wider audiences.

You mean illogical right? Most franchise you look at will show stable sales on very different HW sales so you are basing this on thinking there is any direct correlation that you can make and not only supositions.

I do think there are more variables to the amount of games sold on one console but to refute userbase is illogical. There will be games that will certainly sell to few people regardless of the userbase just like there are exceptions like Mario Kart WiiU and Halo 3... but what is the reason most multiplats sell better on the larger userbase? I know perfectly there are exceptions but to say userbase has no effects on sales is wishful thinking.  

No, totally logical. I talk about facts not about if same game or franchaise have stable sale. Like I wrote: game will sell better on bigger instal base than on smaller, clear fact. Imagine that PS4 has instal base of around 20m, logical assumption (and following current attachment rate for that specific game) would be that U4 would sell around 2m instead of 4m on 40m instal base. There is nothing illogical here.

I already wrote that comparing sales of two games on two platforms where one platform have twice bigger instal base than other doesn't make any sense.

Nah that's not 5 million sell through. It's 5 million total including shipped to retail and sell through for digital. Stop lying azzanation, the 5 million figure originated on NeoGAF from a 343I dev and he never clarified sell in or sell through despite being asked multiple times. Aside from that, ZHugeEX himself agreed that the numbers are sell in. All the articles stating halo sold 5 million use that one NeoGAF post as their source since no official Microsoft PR was ever released on the subject.

Okay so what kind of impact does releasing a game during Christmas vs releasing a game in May have? Or does that not count either.

I really wouldn't have expected people to be this defensive about a sales comparison, who gives a fuck. This is the worst selling halo so far, against the best selling uncharted ever with the latter outpacing the former in half the time. It is what it is.

While install base should have some kind of an effect, you're likely dramatically overestimating it in the bolded. There are many other variables, some of which are often far more significant than install base. 

Why has Pokemon X/Y sold 15+ million units, rather than 6 - 7 million?
Why did Halo 3, Reach, and 4 all have similar launches, despite dramatically different install bases at each time? 
Why is Black Ops 3 on the PS4 seemingly doing better than any previous version of CoD? 
Why is God of War 3 the highest selling entry in the series, despite half the number of systems? 
Why did Forza Horizon 3 launch better on the X1 than FH2 did on the X1 + 360?
Why do individual game sales not increase with install base?
Why, despite double sales and seemingly similar attach rates, do some series sell less than double on the PS4?
Why did the PS4 version of FIFA15 sell 3:1 versus the X1 version?
Why did Destiny sell more on the PS4/X1 than PS3/360?
Etc

Some of these questions are easier to answer than others, but all are down to variables outside of install base. Often they can make install bases seem almost irrelevant (even if that's not the case).

Like i said, install base no doubt does have some kind of effect, but linear extrapolation isn't logical at all. There are too many other variables, most on which in UC4's case work in its favour (pre-existing fanbase, marketing, etc), that throw the math off.

You, apparently, judging by all the posts on this page alone. Why so defensive about people discussing sales? I really wouldn't have expected people to be this defensive about a sales comparison.

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I care more when people deliberately spread misinformation than I do about a halo vs uncharted sales comparison tbh. But because I feel comfortable in my knowledge about video games or sales, I'm more inclined to visit these threads and speak out when I feel that I can bring relevent information to the table, such as the tweets regarding this very topic. I could not care less if uncharted sold more or less than halo because halo is irrelevant to me