Seasonal changes refer to social and economic phenomena affected bynatural conditionAnd social customsPeriodicityVolatility.For example, the production of most agricultural and sideline products has off-season and peak season due to seasonal change, so agricultural and sideline products are produced by raw material processing industry, and the purchase and sale of agricultural and sideline products in the trade system and the freight volume of the transportation department also have seasonal changes.[1]
Due to the influence of natural conditions and social factors,objectiveThe statistical value of the phenomenon changes regularly within a year.Due to the influence of natural conditions and social factors, the statistical values of objective phenomena change regularly within a year.Such as crop yield and many suppliessales volume, both of which are seasonal.Studying the seasonal changes in the past is of great significance for determining the current operation and management activities, especially for organizing business activities, avoiding adverse effects caused by seasonal changes, forecasting the future and making plans[2]。
classification
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influencetime seriesThe factors of change can generally be divided into three categories:
1、Are purely time factors, such as seasonal factors related to climate, calendar days, holidays, etc. (expressed in S);
2、It is a trend factor related to different development stages (represented by T);
3、yesEconomic variablesRegular interaction. This interaction develops regularly with the extension of time. It is shown in the time series that there are regular periodic fluctuations (expressed in C).
In addition, there are also some accidental factors, such as social psychology, political factors, etc. (expressed by I).Therefore, the time series can generally be expressed as: Y=T × C × I × S.In order to reflect the essence of objective economic phenomenon more accurately, we must eliminate and adjust the seasonal factors in advance.
measuring method
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In order to reduce the impact of special circumstances in each year, the seasonal variation is generally measured based on data of at least five years.Common determination methods include:
The simplest method, which is characterized by that the influence of long-term trend is not considered when determining seasonal variation.
The basic steps are as follows: ① calculate the average of the same month in several years; ②Calculate the total monthly average in several years; ③Compare the average of the same month in several years with the total monthly average, and calculate the ratio of each month from January to December, that is, the seasonal ratio (percentage), which indicates the degree to which the average level of each month is higher or lower than the total average level of the whole period, that is, it reflects the general law of seasonal changes.The sum of the seasonal ratio of 12 months should be equal to 1200%, but the calculation is often slightly different due to the choice of mantissa, so the difference needs to be apportioned to each month, that is, an adjustment coefficient should be used to multiply the seasonal ratio of each month.The adjustment factor is 1200 divided by the sum of seasonal ratios of each month.The adjusted seasonal ratio is the final seasonal index used to reflect seasonal changes[3]。
2. Moving average trend elimination method
It is a method to eliminate the influence of long-term trend and then calculate the seasonal index.
The basic steps are as follows: ① Calculate the trend value of each month according to the monthly data of each year, and the method is to calculate the moving average by moving 12 months; ②Divide the actual value of each month by the trend value to get the value after eliminating the trend; ③Average the values of the same month of each year after removing the trend, and the average is the seasonal ratio of each month.Similarly, if the sum of seasonal ratios of each month is not equal to 1200%, it needs to be adjusted according to the above adjustment method, and the seasonal index will be finally obtained after adjustment.
