an> <span class="p">(</span><span class="n">df</span><span class="o">.</span><span class="n">price</span><span class="o">.</span><span class="n">std</span><span class="p">())</span>
    <span class="n">lower</span> <span class="o">=</span> <span class="n">df</span><span class="o">.</span><span class="n">price</span><span class="o">.</span><span class="n">mean</span><span class="p">()</span> <span class="o">-</span> <span class="n">n_std</span> <span class="o">*</span> <span class="p">(</span><span class="n"