_________ component of time series is indicated by periodic variation year after year.

Answer

To solve the question regarding the component of a time series that is indicated by periodic variation year after year, we can follow these steps:

### Step 1: Understand the Components of Time Series
Time series data can be broken down into several components: trend, seasonal, cyclical, and irregular. Each of these components has distinct characteristics.

**Hint:** Remember that time series components represent different patterns in data over time.

### Step 2: Identify the Nature of the Variation
The question specifies "periodic variation year after year." This indicates that the variation occurs at regular intervals, specifically on an annual basis.

**Hint:** Think about what type of variation recurs at fixed intervals, such as seasons or specific times of the year.

### Step 3: Relate the Variation to Time Series Components
Among the components of time series, the "seasonal component" is characterized by regular fluctuations that occur at specific intervals, such as quarterly, monthly, or yearly. Since the question refers to yearly variations, it aligns with the definition of a seasonal component.

**Hint:** Consider how seasons affect various phenomena, such as sales, weather patterns, etc., which recur annually.

### Step 4: Conclude the Answer
Based on the understanding of time series components and the nature of the variation described in the question, we can conclude that the answer is the "seasonal component."

**Final Answer:** The seasonal component of time series is indicated by periodic variation year after year.