I was trying to make up my mind whether to use "Login" or "Sign In" in the project I am working on by going through various well known websites. In my project, user can log in using either Facebook or email id. I landed upon Duolingo's and saw this:
Now, what? Why not "Login with Facebook or Login using Facebook? Why have they used different terminologies to essentially depict one thing? Or am I missing something here?
This is more of a grammatical question than UX, but I think i have the answer you're looking for.
Login should be used as a noun, as in "use your login information to access our services". The example you provide actually incorrectly uses the word "login" in 2 locations (they should have separated it into 2 words - "log" and "in").
Don't ever use signin - not a word.
Log in and Sign in are both appropriate call to actions for this usage. Both are considered verbs when used in this context, prompting an action from the user. In my opinion, use log as opposed to sign. Hope this helps and that it was what you were looking for.
This is generally just based on the user's preference, but I would say you would be better off using sign in, but be consistent.
You see, 'log in' does not generally mean enter the system with credentials, logically, while 'sign in' is more recognizable. Take for example you are going into a hotel, and you need to get a room. You use your credentials (Your ID, Credit/Debit card, etc) to get the room, and when you get to the front desk, they sign those credentials into the hotel's database/log. I may be the only one, but log in is generally more confusing, and does not make as much sense as sign in.
Then there comes consistency; you don't want to use something that is not widely used, as it will be overall more confusing and unacceptable compared to the majority of services. Take for example this chart:
Quite easily, you notice that 'Sign In' is very much used, as opposed to log in, login, logon, or another word/phrase. This is generally leaning you in the direction to use Sign In because it will be something that people will recognize faster, and it just makes more sense generally and logically.
It is up to you in the end, but I would generally say that Sign In is the better option, given the fact that it has already found it's place in the computing world as the main way of saying 'enter the site/service here!', and it is more logical if you think about it.
I hope this helps, as it is more on the confusing part of UX design.
If you are leveraging Facebook then you want to align the labeling with that system. They use "Log In".
This is not so much a semantic issue as it is a design consistency issue. Users (should) understand that they need to enter credentials. Consistency is key to avoiding ambiguity and creating the appearance of a unified system - leading to your goal: getting the user past the Log In with as little friction as possible.
What is the action that these verbs or phrases stand for? (Sometimes nouns are used, too.)
Enter your personal credentials, including some unique ID like your email address or a user name and a password, to create a new local account (for FREEE!!) that somehow personalizes the site to your needs or gives you more rights, e.g. posting comments.
join; sign up; subscribe
buy membership, become a member, get your membership …
try it; free trial; get involved
create account, create user, create profile
new user, new member, new author, new account, new profile
I’m new here
Enter your personal credentials to activate your local account that was created earlier.
enter; log in (or login), log on; sign in, sign on; go in
my account, my profile, my Sitename_; account, profile; me, I
members area, members only, private area, protected area, pay area; intranet
Sitename Pro, Sitename Plus
let me in!
I’m a member, I’m a subscriber, I have a subscription
Let the site access your global account so you do not need a local one. You may have to enter your credentials for the global identification service provider (Facebook, Google, Twitter; OpenID/OAuth …) if you are not permanently “on”.
auto login, quick login, shared login
Let the site use your public account to create a local account. (Beware of anti-patterns!)
link account, share account
Let the site use your public account to activate your linked local account.
sign in; log in
So, for most cases, log in and sign in are either synonymous or not applicable. The exemption is 3., signing in with the help of third parties, where one should not use log in without qualifier. One might want to avoid such ambiguous terms which also sound very technical, but they are also so common users might expect them.
Actual terms used is not important, the difference in terms indicate different workflow, namely "Authenticate" vs. "Trust"
A user is frequently already "Logged In" Google or Facebook accounts. As these accounts are already authenticated (i.e. you have proven who you are) then a user does not have to "Log In" again. All that is required is to tell Duolingo which account to trust.
Does using different terminology actually help the user? Possibly as the user will experience different flow, and are hinted they won't need to "Log In" to Google or Facebook accounts again.
However...Twitter makes the distinction between 'Signing Up' and 'Logging In'. Perhaps the different terms help to differentiate the actions? I would think that the elements are distinct enough by themselves (colour, layout etc.). Using both terms apparently has its place, but I would suggest not in the example you gave where there doesn't seem to be a valid reason for mixing them.
As a non-native speaker I always found the "Sign In - Sign Up" pair confusion. It makes me think, even after 10 years of living in english speaking I still have to pause for a split of a second to map them on the old Register and Login. On the other hand if one of the pair is either Register or Create account or something I don't need to think about the other verb.
I wonder if there is any research on how non-native speakers perceive this or of it is just my dyslexia.
Highly the simple answer it lies with Content Strategy to define the term.
Difference between log in and Sign-in could be something System v Personified. So a system like intranet application in a retail store may say "LOGIN" to define it in line with all users are already tagged to the application, which exist in the database and always had to be logged in to use the system. Now if you would traverse into B2C sites the same login could be used in Banking applications where your usage will be recurring by getting you an account and more of approaching towards a system.
When its personified as what sign in means to be "sign-in on a register as arriving a hotel" (defined in dictionary). This is more often seen a one-time or sub-set of LOGIN (as found in Duolingo). Log in works to be paramount common term, but when invitation is more personified with a human touch it just means to be Sign-in.
Sign-in as a term evolved by simply signing up yourself in physical world for the service, which actually a metaphor versus in a digital world the log in is more attuned towards the system and creation of database.
So are there any differences between these two? Yes, only on softer aspects though both do the same job - whenever the usage is permanent, system driven, rigid, formal it takes the route of Login. While to entice users, greet, nudge, personalize or quite casual then the Sign in is commonly used. But it does not mean log in cannot be replaced in places of Sign in.
Examples could be if on a Banking or Trading site if had to use the term "Sign in" may be inappropriate vs. Social sites, Music channel etc.
It boils down to tone, voice that is intended for the context!
Log in distinctively gives a feel of entering a computer or digital world. Wiki tells me, it started appearing in all bulletin boards around 1970. Probably then it made sense. Today this is not the case.
Internet and computer applications have become very common and day to day part of lives for many of us. As computers and internet became mainstream, we started to make it more friendly and approachable. A move from logging in which appears technical to a friendlier sign in, like in a hotel indicates the same.
As you are looking for studies and more credible sources, Homepage Usability by Jakob Nielsen (together with Marie Tahir, 2002, p. 53) has more information. Another answer has already pointed that out.