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The way the two assumptions are related is that the period assumption upholds relevance.
The period assumption requires reports to be prepared for a period of time, in order to obtain comparability of results. If the assumption was not adopted, this would mean that the business would continue indefinitely (due to the going concern assumption), and therefore we would never be able to calculate profit and prepare reports such as the Income Statement/Cash Flow Statement. Furthermore, it ensures that only revenue earned and expenses incurred for the period are included, and allows as to distinguish between our current and non-current assets/liabilities.
All of this information is capable of making a difference to the decisions made by users. For example: if expenses and revenues from other reporting periods were included, this would affect the net profit of the business, and therefore alter perceptions of financial performance. How would a user assess overall expenditure on expenses such as wages or advertising if there was reporting periods? Therefore, the reporting period aids the decision-making process, upholding relevance.