DEFINITION of 'TimePeriod Basis'
An implication surrounding the use of timeseries data in which the final statistical conclusion can change based on to the starting or ending dates of the sample data. The concept discourages using a smaller time series, as chance events are more likely to be reflected in the conclusion.
INVESTOPEDIA EXPLAINS 'TimePeriod Basis'
When testing various investment strategies, the timeseries basis can be significant. Idiosyncratic events or using a time frame that only captures certain business cycles can make certain strategies appear more profitable than they really are. To help avoid the timeperiod basis when testing investment ideas, one should use very long periods of sample data.
For example, let's test a momentum strategy in which an investor enters a long or short position in a stock that has a rapid price increase or decrease. The investor identifies a stock to buy for the testing period. If the strategy is tested during a period where the entire market was strengthening, it could make the momentum strategy appear very profitable. However, if the profits were driven only by the conditions of the market, then the strategy would become dangerous in an economic downturn. Extending the testing period to allow full business cycles would reveal the true profitability of the strategy.

Business Cycle
The fluctuations in economic activity that an economy experiences ... 
Unsystematic Risk
Company or industry specific risk that is inherent in each investment. ... 
Idiosyncratic Risk
Risk that is specific to an asset or a small group of assets. ... 
Momentum
The rate of acceleration of a security's price or volume. The ... 
Time Horizon
The length of time over which an investment is made or held before ... 
Sharpe Ratio
A ratio developed by Nobel laureate William F. Sharpe to measure ...

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