To ensure a smooth visualization process, data-sets must be clean. "Data cleaning" refers to the process of standardizing the data-set. Entries must be spelled correctly, labeled and formatted the same throughout the entire data-set in order to avoid processing errors. Data cleaning can be tedious and time-consuming to do by hand. Sites such as DataWrangler or Open Refine (formerly Google Refine) make the task easier by helping with the data cleaning process.