Does sending a thank you email after interview increase chances?

I have an Excel spreadsheet for all interviews and I put down a comment if I sent that email. In some cases I did write that email, but in other cases I did not. My thank you emails are more detailed, normally, they also have information I forgot to mention during the interview. Post interview I sit down in a cafe to cool down and write down everything that was discussed. Then I summarize this and write that email.

In the cases I wrote the thank you email I had, as a rough estimate, a 20% higher chance of getting a job offer.

This is only a personal spreadsheet, so not really meaningful.

I immediately delete thank you emails.  I don't care about them at all.  If you were interviewing with me I wouldn't care if you sent one or not.  If anyone is deciding a hiring decision based on a thank you email then they're an idiot.  That said, there are a lot of idiots in banking so you might as well send one, just don't put too much thought into it.

Whenever I interview a kid, I've made my decision far before the thank you email, so no it doesn't affect my decision, and as a rule of thumb I never respond to thank you emails to avoid giving kids false hope if I decided to not push them forward in the process. However I still think thank-you emails are generally customary and you should always send one to each interviewer to be safe.

I’d say its less of a boost to your chances and more of a common practice. It probably won’t help you to include one (as it is expected), but it could harm you if you do not.

Echoing the above comment, it won’t help you in most cases, but not sending one or probably worse, sending a bad one, can DEFINITELY hurt you. Just send a generic one with one sentence vaguely referencing something specific about your conversation with the interviewer. Do not send the exact same email to all superday interviewers, at least put in the effort to throw something slightly different in each.

In my experience thank you emails are important.  I agree with other posters in that they won't help you get the job, but it could hurt if you don't send one since some people expect them.  They cost you nothing and don't have to be complicated.  I was involved in the hiring process at a prior firm, and on at least one occasion the deciding factor for the last slot in the second round between two otherwise comparable candidates was that one had sent a thank you email and the other had not.  That said, we would not have cared if our top candidates didn't send them.

In my own recruiting experience, I was once late in a process where I had half a dozen 1:1 calls with different members of the team.  I sent thank you emails to everyone except the last person I spoke to (was a Friday afternoon and I was lazy and wanted to start my weekend) and the recruiter called me the next day wanting to know what was going on because the company thought I might be losing interest in the position since I didn't send a thank you email.  So it's definitely something some firms will notice.  Maybe more important for smaller teams.

Again, just my experiences - your mileage may vary.

If there is time between rounds or after the Superday, a handwritten thank you note has worked great for me in the past. I've had several people either call or email me after to let me know how impressed they were. It's a little tricky with covid since many people aren't in the office, but taking the time to write them out can make a difference if it's a toss-up between you and someone else.