Markov Chain

\usetikzlibrary{automata, positioning}

\begin{tikzpicture}[shorten >=1pt, node distance=2.5cm, on grid, auto]
  \node[state] (A) {$A$};
  \node[state] (B) [right=of A] {$B$};
  \node[state] (C) [right=of B] {$C$};

  \path[->]
  (A) edge[loop above] node{0.5} ()
  edge[bend left] node{0.5} (B)
  (B) edge[bend left] node{0.3} (A)
  edge[bend left] node{0.7} (C)
  (C) edge[loop above] node{0.2} ()
  edge[bend left] node{0.8} (B);
\end{tikzpicture}