title: Logistic growth in R and TI 84 gave different results
body: <p>I have the following data:</p>
  
  <pre><code>x &lt;- c(10,20,30,40,50,60,70)
  y &lt;- c(110,235,435,655,830,930,980)
  population &lt;- data.frame(y,x)
  </code></pre>
  
  <p>When I try to compute the parameters of a logistic growth model:</p>
  
  <pre><code>fit_data &lt;- nls(y ~ SSlogis(x, phi1, phi2, phi3), data = population)
  alpha &lt;- coef(fit_data)  #extracting coefficients
  </code></pre>
  
  <p>I obtain the following coefficients:</p>
  
  <pre><code>    phi1 
  1015.103 
      phi2 
  33.29877 
      phi3 
  11.11867 
  </code></pre>
  
  <p>Which are very different from those obtained using a TI-84 calculator: <code>1015.1, 19.98, -0.09</code>. Anyone can explain this difference? Am I doing something wrong with R?</p>