Copyright law varies from country to country, but the best way recognized by everyone is to sign a contract that includes what will be used from the party that will provide you with the statistics.

This is not legal advice, because it depends on a lot of information that is missing, but just to give you a general understanding:
Citing/referring to the source of the information does not mean it is ok for you to use that information - even if the information is "public". If I leave my car out in a public parking lot, it is still mine and leaving it there doesn't mean that you can use it. The same applies to me putting something on the internet. Even if it's public (like on Google), doesn't mean I am allowing you to use it. Each piece of information (statistics, pictures, music...) is owned by someone, and either they offer a license which tells you how you can/can't use it, or you have to contact them and get their explicit permission.

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