The Original Novels both of these stories are based on were written by Ira Ishida. There are a lot of similarities in terms of atmosphere and setting, by the method of which the districts of Ikebukuro and Akihabara are given unique personalities. Both of these stories are urban mysteries where the protagonists investigate various crimes occurring within the city.

They're both Mystery drama's which take a lot of inspiration from the Trick series. They involve a pair of quirky and eccentric characters solving and disproving seemingly supernatural crimes. The crimes and relationship between the two leads are very similar as well as the type of humor.

Psychometrer Eiji is one of the dramas that Tsutsumi Yukihiko directed before Keizoku. Its also the drama that influenced the creation of Keizoku the most. Psychometrer Eiji is almost like a prototype for Keizoku and you can see the characterization and plot points that would be reused.

They're both Buddy Cop dramas with male and female leads investigating weird, abnormal cases. Female leads are both weird and quirky and decided to become police officers to follow in their fathers foot steps. Male leads are both disillusioned with the criminal justice system. The writing and character dynamics are very similar.