Abstract: Radio environment plays an important role in radio astronomy observations. Further analysis is needed on the time and intensity distributions of interference signals for long-term radio environment monitoring. Sample variance is an important estimate of the interference signal decision threshold. Here, we propose an improved algorithm for calculating data sample variance relying on four established statistical methods: the variance of the trimmed data, winsorized sample variance, median absolute deviation, and median of the trimmed data pairwise averaged squares method. The variance and decision threshold in the protected section of the radio astronomy L-band are calculated. Among the four methods, the improved median of the trimmed data pairwise averaged squares algorithm has higher accuracy, but in a comparison of overall experimental results, the cleanliness rate of all algorithms is above 96%. In a comparison between the improved algorithm and the four methods, the cleanliness rate of the improved algorithm is above 98%, verifying its feasibility. The time-intensity interference distribution in the radio protection band is also obtained. Finally, we use comprehensive monitoring data of radio astronomy protection bands, radio interference bands, and interfered frequency bands to establish a comprehensive evaluation system for radio observatory sites, including the observable time proportion in the radio astronomy protection band, the occasional time-intensity distribution in the radio interference frequency band, and the intensity distribution of the interfered frequency band.