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Time variance is the ability to remember historic perspectives. The requirement is to be able to know how something was classified or who owned something and how this changed as time passed.
Understanding time variance
One required feature of a data warehouse is for it to be "Time Variant", giving us the ability to store data representative as it existed at many points in time in the past. Granularity of time variance and the tracking of changing data over time depends upon the needs of the business and can range from representing data values at the time they are changed on up to any periodic inspection/detection interval (hourly, daily, weekly, monthly).
Time variance allows us to join transaction data to reference data that contains values representative of the time of the event. For example; When a customer changes their address in a sales system, that sales system would just update the address. A data warehouse though, would have processes that detect that change and then track that change allowing us to see the customers address before and after that change. This allows us to then tie any sales events to the customer information that was active at the time of the sales event. This would allow you to report sales totals by city of where the customers lived at the time of the sale, as opposed to sales by city based on where everyone currently lives.
Time variance also allows us to adapt to future changes and accurately report information when data changes in the future. For example; If we use a sales type code of 45 that would mean "Sales to General Public" on our sales orders, any future changes to how we describe that code might change the context of past orders. Consider if we change the code 45 another meaning to "Sales to Free Membership Individuals". Does that mean that any past sales orders with sales type code 45 are not "Sales to General Public" and are now "Sales to Free Membership"? Having time variance in place, allows the flexibility to either apply the description of the code active at the time of the sale or just use the new code to categorize all past sales.