guys, I just started homebrewing and I want to know what are the essential data one should collect when brewing beer other than hop schedules and gravities. What additional statistics do you suggest I should focus on which will be helpful for the longer term.
The recipe: grain bill, hops and their times, adjuncts and their times, amount of water. That way you can try to recreate the same beer if it was good. You can tweak any of grain bill, hops, timing, and so on to see how the beer changes.
Once you get more advanced, take other readings: You already mentioned gravity. When you brew all-grain, you can take that at different stages, for example, at the beginning and at the end of lautering to see how gravity changes from first runnings / vorlauf to last runnings. You can use that data to check how good you lautering work is. At some point ph readings may be interesting, to know if you want to change the ph of your water, e.g.,by adding acid malt.
On the process, you can check how fast you can heat up your water (especially when doing multi-step mashes). That will help in planning or give you an idea whether you need a bigger burner.
Here's a couple reading I find important that many over look.
All-grain. Mash pH is very important.
Doing an iodine test during the mash is a good practice to check if conversion is done. Why mash for 90 min if it's done at 45?
Water chemistry. Most home brewers build thier own profile from RO. Then many have great local water. Knowing what's in it will help brewing, as different ions have a big effect on flavor and reaching your correct mash pH.
Record all Alpha Acid % of the hops so you can correct the AA from brew to brew. Each harvest varies in AA.
Update: Almost forgot Chill time. This is very important for late hop additions. Isomerization will continue until the wort is below 175°F. So you should include this chill time to your total hop addition time. Additions that have 45 minute or more are unaffected since isomerization will have completed. But those late aroma and flavor additons may bitter too much if chill time isn't included.
Building off other answers I'd add measuring and recording accurate volumes is key to either all-grain or extract.
Probably the most important thing is that you remember to take notes at all. Getting into the habit of taking notes is a best practice regardless of which data you collect. As you grow as a brewer you might find certain data points more important than the data others collect. It isn't always universal. Maybe your water is nearly perfect for the beers you want to brew so pH isn't that big a deal. Maybe you are a brewer who always makes the same basic beer and then gravity isn't that important to you. However, maybe you really like to try the same basic ingredient but like to try different maltsters. So that info is something you should write down.
Taking the notes on every batch is far more important than remembering to collect the "right" data every once and a while. Helps you track and make adjustments.