Consider the user ratings matrix which consists of $4$ users and item rating given in terms of Likert scale for $4$ books":

$\backslash$table$[[,\backslash$table$[[$Harry$],[$Potter$]],\backslash$table$[[$Alice in$],[$Wormerland$]],\backslash$table$[[$The$],[$secret$]],\backslash$table$[[$The$],[$Alchemist$]]],[$User $1,5,4,5,3],[$User $2,4,\mathrm{.}2,2,4],[$User $3,3,4,4,2],[$User $4,5,3,4,?$