Edward
Yes Wikidata has a mechanism for adding qualifiers and references to claims.
Where there are multiple values we can also mark one of them as preferred and then simple queries will just get the preferred value.
For population figures each value would typically have qualifiers for
* Point in time (when the population figure applies)
* Determination method (census, estimate, whatever)
* Applies to part (total, black, white, christian, muslim etc.)
And each value would have a reference with the organisation, the url, the date the info was published, the date the info was retrieved etc.
The most current total value would be marked as 'preferred'. Values which are widely known but considered unreliable would be included but marked 'deprecated'.
All these properties have already been implemented and we are starting to add them to items.
Hope this helps
Joe