\documentclass{article}
\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usepackage[landscape]{geometry}
\usepackage{fullpage}
\usetikzlibrary{shapes,arrows}
\usepackage{atbegshi}% http://ctan.org/pkg/atbegshi
\AtBeginDocument{\AtBeginShipoutNext{\AtBeginShipoutDiscard}}
\begin{document}
\pagestyle{empty}
% Define block styles
\tikzstyle{decision} = [diamond, draw, fill=blue!20, 
    text width=6em, text badly centered, node distance=3cm, inner sep=0pt]
\tikzstyle{block1} = [rectangle, draw, fill=red!20, 
    text width=40em, text centered, rounded corners, minimum height=4em]
\tikzstyle{block2} = [rectangle, draw, maximum height=1cm, fill=blue!20, 
    text width=10em, text centered, rounded corners, minimum height=4em]
\tikzstyle{block3} = [rectangle, draw, fill=green!20, 
    text width=7.5em, text centered, rounded corners, minimum height=4em]
    \tikzstyle{line} = [draw, -latex']
    \tikzstyle{cloud1} = [draw, rectangle,fill=yellow!20, text width=15em, node distance=8cm, minimum height=6em]
    \tikzstyle{J} = [rectangle, draw, fill=gray!20, 
    text width=10em, text centered, rounded corners, minimum height=1em]
    \tikzstyle{black} = [rectangle, draw, fill=yellow!20, 
    text width=5em, text centered, rounded corners, minimum height=1em]
\begin{tikzpicture}[node distance = 2cm, auto]
    % Place nodes
    \node [block1] (izenburua) {BATEZBESTEKOARI BURUZKO FROGAK};
    \node [J,left of=izenburua,node distance=10cm] (izen) {Josemari Sarasolak sortua, Tex eta Tikz erabiliz, 2016an.};
    \node [block2,below of=izenburua] (ho) {$H_0:\mu=\mu_0$};
    \node [decision,below of=ho,node distance=3cm] (pop normala) {Populazioa (eredua) normala al da?};
   \node [decision,below of=pop normala,left of=pop normala,node distance=3.75cm] (sigmaezag) {Populazioaren desbideratzea ($\sigma$) ezaguna al da?};
   \node [black,right of=pop normala,node distance=3.75cm,xshift=-0.95cm,yshift=-1.25cm] (black) {$n>30$ izan behar da};
   \node [decision,below of=pop normala,right of=pop normala,node distance=3.75cm] (sigmaezag2) {Populazioaren desbideratzea ($\sigma$) ezaguna al da?};
   \node [block2,below of=sigmaezag,node distance=3.5cm] (stxano) {$\hat{s}$ kalkulatu};
   \node [block2,below of=sigmaezag2,node distance=3.5cm] (stxano2) {$\hat{s}$ kalkulatu};
   \node [block3,below of=sigmaezag,left of=sigmaezag,node distance=6cm,xshift=1.5cm] (normal1) {$\overline{x} \sim N(\mu,\frac{\sigma}{\sqrt{n}})$};
   \node [block3,below of=stxano2,node distance=2.5cm] (normal2) {$\overline{x} \sim N(\mu,\frac{\hat{s}}{\sqrt{n}})$};
   \node [block3,below of=stxano,node distance=2.5cm] (t) {$\frac{\overline{x}-\mu}{\hat{s}/\sqrt{n}} \sim t_{n-1}$};
    \node [block3,below of=t,node distance=2cm] (z) {$\frac{\overline{x}-\mu}{\hat{s}/\sqrt{n}} \sim N(0,1)$};
    \node [cloud1,left of=stxano,xshift=1.7cm] (kalkulu) {$\hat{s}=\sqrt{\frac{\sum (x_i-\overline{x})^2)}{n-1}}=\sqrt{\frac{\sum (x_i^2-n\overline{x}^2)}{n-1}}$};
    \node [block3,below of=sigmaezag2,right of=sigmaezag2,node distance=6cm,xshift=-1cm] (normal3) {$\overline{x} \sim N(\mu,\frac{\sigma}{\sqrt{n}})$};
    % Draw edges
    \path [line] (ho) -- (pop normala);
    \path [line] (pop normala) -- node[below,near start,xshift=0.3cm] {Bai} (sigmaezag);
    \path [line] (pop normala) -- node[below,near start,xshift=-0.3cm] {Ez} (sigmaezag2);
    \path [line] (sigmaezag) -- node[below,near start,xshift=0.4cm] {Bai}(normal1);
    \path [line] (sigmaezag) -- node[midway] {Ez}(stxano);
    \path [line] (sigmaezag2) -- node[midway] {Ez}(stxano2);
    \path [line] (sigmaezag2) -- node[midway] {Bai}(normal3);
    \path [line] (stxano) -- (t);
    \path [line] (stxano2) -- (normal2);
    \path [line] (t) -- node[midway] {$n>30$} (z);
    \path [line,dashed] (stxano2) -- (stxano);
    \path [line,dashed] (stxano) -- (kalkulu);
\end{tikzpicture}
\end{document}