This question utilizes the Earned Schedule (ES) method, which is an extension of the traditional Earned Value Management (EVM) framework. While traditional EVM measures schedule variance in currency (dollars/units), Earned Schedule measures it in units of time.

According to the PMI Practice Standard for Earned Value Management and references in the PMBOK Guide:

Identify the Variables:

Earned Schedule (ES): 16 weeks. (Note: In this specific calculation context, 'ES' refers to Earned Schedule---the duration that should have been taken to achieve the current earned value---rather than 'Early Start').

Actual Time (AT): 12 weeks.

Schedule Performance Index (SPI): 1.3 (given).

Formula for Schedule Variance (Time):

The formula for Schedule Variance in terms of time ($SV_t$) is:

$$SV_t = ES - AT$$

Substituting the given values:

$$SV_t = 16 - 12 = 4$$

Validation with SPI:

The formula for the Schedule Performance Index in terms of time ($SPI_t$) is:

$$SPI_t = ES / AT$$

Substituting the values:

$$SPI_t = 16 / 12 = 1.33...$$

This matches the provided SPI of 1.3 (rounded to one decimal place), confirming that the interpretation of the variables is correct.

Conclusion:

A positive Schedule Variance of 4 indicates that the project is 4 weeks ahead of schedule. This is consistent with an SPI greater than 1.0 (1.3), which denotes efficient schedule performance.