1. Introduction

2. Observations and Data Reduction

3. Classification of Variables

4. Structure Function of the Variations
4.1. Measurement errors
4.2. Structure Function Correction and Fitting
4.3. Variability Timescales
4.4. Comparison of SF with the visual variability classification

5. Interpretation as Interstellar Scintillation
5.1. Galactic Dependence of ISS
5.2. Dependence of ISS on Source Spectral Index and Flux Density
5.3. Source Models
5.4. Combined effects of Intrinsic Variations and ISS
5.5. Dependence of ISS on Source Redshift
5.6. Intermittent Variability

6. Conclusions