It’s also important to mention that weak vowels can often be used interchangeably in unstressed syllables, especially /ə/ and /ɪ/. When this happens we say the they are in free variation. So, you can use the one that suits you best, and you’ll always be right.
The word animal can be pronounced /ˈænɪməl/ or /ˈænəməl/ without much difference. Similarly, family can be pronounced /ˈfæmɪli/ or /ˈfæməli/, and also /ˈfæmli/. This last process is called compression (and it’s also the subject of a future article in this website).
A word like signature is particularly interesting. Here too we can use the phonemes /ə/ and /ɪ/ interchangeably in the second syllable (/ˈsɪɡnətʃə/ or /ˈsɪɡnɪtʃə/) and neither of them coincides with the idea a Spanish speaker normally has of how to pronounce this word (usually with an /a/ in the second syllable and an /u/ in the third, something like */ˈsiɡnatʃur/).
Another example: the Oxford online dictionary gives the transcriptions /ˈkrɪmɪnl/ and/ˌkrɪmɪˈnæləti/ for criminal and criminality; on the other hand, Longman agrees on /ˈkrɪmɪnəl/, but provides a slightly different version in /ˌkrɪməˈnæləti/.
Also, for the same reason, the suffix -ity, which is found in so many words (quality, velocity, charity, etc.) can be pronounced /əti/ (more common nowadays) or /ɪti/ (a bit on the way out).
Notice how clearly the first option, /əti/, can be heard in this example:
Now, listen to the same speaker, just a few minutes later, in this recording:
Oh, yes… Clarity, fluidity, that rumbled in the basses. Well, Britten’s dawn from Edward Gardner and the BBC Philarmonic. It’s a very good recorded sound as well this one, isn’t it? (Andrew McGregor, BBC3).
As you can appreciate, Andrew McGregor pronunces /ˈklærəti/ but /fluˈɪdɪti/. So, there’s nothing wrong with alternating the two possibilities.
This type of variation also takes place between /ʊ/ and /ə/. The verb speculate can be said /ˈspekjʊleɪt/ or /ˈspekjəleɪt/, and both are completely acceptable (as in the previous examples, the option with the schwa displays a higher degree of reduction). There are many other cases in which the combination of phonemes /jʊ/ becomes /jə/ in unstressed syllables.
What you have to remember about these small differences is that their importance is really negligible. Both options are correct and speakers can do as they please.