Decision Tree - How to get good estimates - data-mining

Question

I want to examine a data set of a web shop. The data set includes the number of visits and the number of orders including some personal data.
I want to find out, on which values a "high order rate" depends. My idea was to use a decision tree for that because the rules are easy to understand and you can easily take action according to the results.
I created a new column called "abortion rate". Its the relation of visits and orders. Everything above 50 % is high, the others are low.
After that I split the data in training and test data and generate a model based on a decision tree.
My problem is, that there are just "a few" records with a low abortion rate. The decision tree seems to work fine but actually it declares nearly all records as high, just 5 records with low abortion rate are correct.
I don't really know how to figure this out. Any ideas?
Thanks in advance.
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<process version="5.3.015">
<context>
<input/>
<output/>
<macros/>
</context>
<operator activated="true" class="process" compatibility="5.3.015" expanded="true" name="Process">
<parameter key="logverbosity" value="init"/>
<parameter key="random_seed" value="2001"/>
<parameter key="send_mail" value="never"/>
<parameter key="notification_email" value=""/>
<parameter key="process_duration_for_mail" value="30"/>
<parameter key="encoding" value="SYSTEM"/>
<process expanded="true">
<operator activated="true" class="read_csv" compatibility="5.3.015" expanded="true" height="60" name="Read CSV" width="90" x="45" y="30">
<parameter key="csv_file" value="C:\temp\WebData.csv"/>
<parameter key="column_separators" value=";"/>
<parameter key="trim_lines" value="false"/>
<parameter key="use_quotes" value="true"/>
<parameter key="quotes_character" value="""/>
<parameter key="escape_character" value="\"/>
<parameter key="skip_comments" value="false"/>
<parameter key="comment_characters" value="#"/>
<parameter key="parse_numbers" value="true"/>
<parameter key="decimal_character" value=","/>
<parameter key="grouped_digits" value="false"/>
<parameter key="grouping_character" value=","/>
<parameter key="date_format" value=""/>
<parameter key="first_row_as_names" value="false"/>
<list key="annotations">
<parameter key="0" value="Name"/>
</list>
<parameter key="time_zone" value="SYSTEM"/>
<parameter key="locale" value="English (United States)"/>
<parameter key="encoding" value="windows-1252"/>
<list key="data_set_meta_data_information">
<parameter key="0" value="ID.true.integer.attribute"/>
<parameter key="1" value="Age.true.integer.attribute"/>
<parameter key="2" value="Sex.true.polynominal.attribute"/>
<parameter key="3" value="Income.true.polynominal.attribute"/>
<parameter key="4" value="PersonsHoushold.true.integer.attribute"/>
<parameter key="5" value="ShippingFlat.true.binominal.attribute"/>
<parameter key="6" value="OneKlickBuy.true.binominal.attribute"/>
<parameter key="7" value="CustomerSince.true.real.attribute"/>
<parameter key="8" value="Visits.true.integer.attribute"/>
<parameter key="9" value="Orders.true.integer.attribute"/>
<parameter key="10" value="att11.false.attribute_value.attribute"/>
</list>
<parameter key="read_not_matching_values_as_missings" value="false"/>
<parameter key="datamanagement" value="double_array"/>
</operator>
<operator activated="true" class="generate_attributes" compatibility="5.3.015" expanded="true" height="76" name="Generate Attributes" width="90" x="179" y="30">
<list key="function_descriptions">
<parameter key="AbortionRate" value="(Visits-Orders)/Visits"/>
</list>
<parameter key="use_standard_constants" value="true"/>
<parameter key="keep_all" value="true"/>
</operator>
<operator activated="true" class="generate_attributes" compatibility="5.3.015" expanded="true" height="76" name="Generate Attributes (2)" width="90" x="313" y="30">
<list key="function_descriptions">
<parameter key="AbortionRate_Bio" value="(if(AbortionRate<0.5, "low","high"))"/>
</list>
<parameter key="use_standard_constants" value="true"/>
<parameter key="keep_all" value="true"/>
</operator>
<operator activated="true" class="set_role" compatibility="5.3.015" expanded="true" height="76" name="Set Role" width="90" x="447" y="30">
<parameter key="attribute_name" value="AbortionRate_Bio"/>
<parameter key="target_role" value="label"/>
<list key="set_additional_roles"/>
</operator>
<operator activated="true" class="select_attributes" compatibility="5.3.015" expanded="true" height="76" name="Select Attributes" width="90" x="581" y="30">
<parameter key="attribute_filter_type" value="subset"/>
<parameter key="attribute" value=""/>
<parameter key="attributes" value="Income|Sex|OneKlickBuy|ShippingFlat|Age|PersonsHoushold|AbortionRate_Bio|CustomerSince"/>
<parameter key="use_except_expression" value="false"/>
<parameter key="value_type" value="attribute_value"/>
<parameter key="use_value_type_exception" value="false"/>
<parameter key="except_value_type" value="time"/>
<parameter key="block_type" value="attribute_block"/>
<parameter key="use_block_type_exception" value="false"/>
<parameter key="except_block_type" value="value_matrix_row_start"/>
<parameter key="invert_selection" value="false"/>
<parameter key="include_special_attributes" value="false"/>
</operator>
<operator activated="true" class="split_data" compatibility="5.3.015" expanded="true" height="94" name="Split Data" width="90" x="715" y="30">
<enumeration key="partitions">
<parameter key="ratio" value="0.7"/>
<parameter key="ratio" value="0.3"/>
</enumeration>
<parameter key="sampling_type" value="shuffled sampling"/>
<parameter key="use_local_random_seed" value="false"/>
<parameter key="local_random_seed" value="1992"/>
</operator>
<operator activated="true" class="decision_tree" compatibility="5.3.015" expanded="true" height="76" name="Decision Tree" width="90" x="447" y="210">
<parameter key="criterion" value="gain_ratio"/>
<parameter key="minimal_size_for_split" value="4"/>
<parameter key="minimal_leaf_size" value="2"/>
<parameter key="minimal_gain" value="0.02"/>
<parameter key="maximal_depth" value="20"/>
<parameter key="confidence" value="0.25"/>
<parameter key="number_of_prepruning_alternatives" value="3"/>
<parameter key="no_pre_pruning" value="false"/>
<parameter key="no_pruning" value="false"/>
</operator>
<operator activated="true" class="apply_model" compatibility="5.3.015" expanded="true" height="76" name="Apply Model" width="90" x="648" y="210">
<list key="application_parameters"/>
<parameter key="create_view" value="false"/>
</operator>
<connect from_op="Read CSV" from_port="output" to_op="Generate Attributes" to_port="example set input"/>
<connect from_op="Generate Attributes" from_port="example set output" to_op="Generate Attributes (2)" to_port="example set input"/>
<connect from_op="Generate Attributes (2)" from_port="example set output" to_op="Set Role" to_port="example set input"/>
<connect from_op="Set Role" from_port="example set output" to_op="Select Attributes" to_port="example set input"/>
<connect from_op="Select Attributes" from_port="example set output" to_op="Split Data" to_port="example set"/>
<connect from_op="Split Data" from_port="partition 1" to_op="Decision Tree" to_port="training set"/>
<connect from_op="Split Data" from_port="partition 2" to_op="Apply Model" to_port="unlabelled data"/>
<connect from_op="Decision Tree" from_port="model" to_op="Apply Model" to_port="model"/>
<connect from_op="Apply Model" from_port="labelled data" to_port="result 1"/>
<connect from_op="Apply Model" from_port="model" to_port="result 2"/>
<portSpacing port="source_input 1" spacing="0"/>
<portSpacing port="sink_result 1" spacing="0"/>
<portSpacing port="sink_result 2" spacing="0"/>
<portSpacing port="sink_result 3" spacing="0"/>
</process>
</operator>
</process>
UPDATE:
I changed my Rapidminer process to this:
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<process version="5.3.015">
<context>
<input/>
<output/>
<macros/>
</context>
<operator activated="true" class="process" compatibility="5.3.015" expanded="true" name="Process">
<process expanded="true">
<operator activated="true" class="read_csv" compatibility="5.3.015" expanded="true" height="60" name="Read CSV" width="90" x="45" y="30">
<parameter key="csv_file" value="C:\temp\WebData.csv"/>
<parameter key="decimal_character" value=","/>
<parameter key="first_row_as_names" value="false"/>
<list key="annotations">
<parameter key="0" value="Name"/>
</list>
<parameter key="encoding" value="windows-1252"/>
<list key="data_set_meta_data_information">
<parameter key="0" value="ID.true.integer.attribute"/>
<parameter key="1" value="Age.true.integer.attribute"/>
<parameter key="2" value="Sex.true.polynominal.attribute"/>
<parameter key="3" value="Income.true.polynominal.attribute"/>
<parameter key="4" value="PersonsHoushold.true.integer.attribute"/>
<parameter key="5" value="ShippingFlat.true.binominal.attribute"/>
<parameter key="6" value="OneKlickBuy.true.binominal.attribute"/>
<parameter key="7" value="CustomerSince.true.real.attribute"/>
<parameter key="8" value="Visits.true.integer.attribute"/>
<parameter key="9" value="Orders.true.integer.attribute"/>
<parameter key="10" value="att11.false.attribute_value.attribute"/>
</list>
<parameter key="read_not_matching_values_as_missings" value="false"/>
</operator>
<operator activated="true" class="generate_attributes" compatibility="5.3.015" expanded="true" height="76" name="Generate Attributes" width="90" x="179" y="30">
<list key="function_descriptions">
<parameter key="A_AbortionRate" value="(Visits-Orders)/Visits"/>
</list>
</operator>
<operator activated="true" class="generate_attributes" compatibility="5.3.015" expanded="true" height="76" name="Generate Attributes (2)" width="90" x="313" y="30">
<list key="function_descriptions">
<parameter key="A_AbortionRate_Bio" value="(if(AbortionRate<0.5, "low","high"))"/>
<parameter key="A_Age" value="(if(Age<18,"Teen",if(Age<30,"young Adult",if(Age<60,"Adult","Pensioner"))))"/>
<parameter key="A_CustomerSince" value="(if(CustomerSince<=1,"up to 1 Year",if(CustomerSince<=2,"1-2 Years",if(CustomerSince<=3,"2-3 Years","over 3 Years"))))"/>
<parameter key="A_PersonsHoushold" value="(if(PersonsHoushold<=1,"Single",if(PersonsHoushold<=2,"Pair",if(PersonsHoushold<=5,"Family","extended family"))))"/>
</list>
</operator>
<operator activated="true" class="set_role" compatibility="5.3.015" expanded="true" height="76" name="Set Role" width="90" x="447" y="30">
<parameter key="attribute_name" value="A_AbortionRate_Bio"/>
<parameter key="target_role" value="label"/>
<list key="set_additional_roles">
<parameter key="K_ID" value="id"/>
</list>
</operator>
<operator activated="true" class="select_attributes" compatibility="5.3.015" expanded="true" height="76" name="Select Attributes" width="90" x="581" y="30">
<parameter key="attribute_filter_type" value="subset"/>
<parameter key="attributes" value="A_AbortionRate_Bio|A_Aage|A_CustomerSince|Income|Sex|OneKlickBuy|ShippingFlat|A_PersonsHoushold"/>
</operator>
<operator activated="true" class="split_validation" compatibility="5.3.015" expanded="true" height="130" name="Validation" width="90" x="715" y="30">
<parameter key="sampling_type" value="stratified sampling"/>
<process expanded="true">
<operator activated="true" class="multiply" compatibility="5.3.015" expanded="true" height="94" name="Multiply (2)" width="90" x="45" y="30"/>
<operator activated="true" class="filter_examples" compatibility="5.3.015" expanded="true" height="76" name="Filter Examples (3)" width="90" x="179" y="120">
<parameter key="condition_class" value="attribute_value_filter"/>
<parameter key="parameter_string" value="A_AbortionRate_Bio = high"/>
</operator>
<operator activated="true" class="sample" compatibility="5.3.015" expanded="true" height="76" name="Sample (4)" width="90" x="313" y="120">
<parameter key="sample_size" value="1675"/>
<list key="sample_size_per_class"/>
<list key="sample_ratio_per_class"/>
<list key="sample_probability_per_class"/>
</operator>
<operator activated="true" class="filter_examples" compatibility="5.3.015" expanded="true" height="76" name="Filter Examples (4)" width="90" x="179" y="30">
<parameter key="condition_class" value="attribute_value_filter"/>
<parameter key="parameter_string" value="A_AbortionRate_Bio = low"/>
</operator>
<operator activated="true" class="sample" compatibility="5.3.015" expanded="true" height="76" name="Sample" width="90" x="313" y="30">
<parameter key="sample_size" value="1675"/>
<list key="sample_size_per_class"/>
<list key="sample_ratio_per_class"/>
<list key="sample_probability_per_class"/>
</operator>
<operator activated="true" class="append" compatibility="5.3.015" expanded="true" height="94" name="Append (2)" width="90" x="447" y="75"/>
<operator activated="true" class="decision_tree" compatibility="5.3.015" expanded="true" height="76" name="Decision Tree (2)" width="90" x="581" y="75">
<parameter key="criterion" value="information_gain"/>
<parameter key="minimal_gain" value="0.025"/>
<parameter key="maximal_depth" value="6"/>
</operator>
<connect from_port="training" to_op="Multiply (2)" to_port="input"/>
<connect from_op="Multiply (2)" from_port="output 1" to_op="Filter Examples (4)" to_port="example set input"/>
<connect from_op="Multiply (2)" from_port="output 2" to_op="Filter Examples (3)" to_port="example set input"/>
<connect from_op="Filter Examples (3)" from_port="example set output" to_op="Sample (4)" to_port="example set input"/>
<connect from_op="Sample (4)" from_port="example set output" to_op="Append (2)" to_port="example set 2"/>
<connect from_op="Filter Examples (4)" from_port="example set output" to_op="Sample" to_port="example set input"/>
<connect from_op="Sample" from_port="example set output" to_op="Append (2)" to_port="example set 1"/>
<connect from_op="Append (2)" from_port="merged set" to_op="Decision Tree (2)" to_port="training set"/>
<connect from_op="Decision Tree (2)" from_port="model" to_port="model"/>
<portSpacing port="source_training" spacing="0"/>
<portSpacing port="sink_model" spacing="0"/>
<portSpacing port="sink_through 1" spacing="0"/>
</process>
<process expanded="true">
<operator activated="true" class="apply_model" compatibility="5.3.015" expanded="true" height="76" name="Apply Model (2)" width="90" x="45" y="30">
<list key="application_parameters"/>
</operator>
<operator activated="true" class="performance" compatibility="5.3.015" expanded="true" height="76" name="Performance" width="90" x="179" y="30"/>
<connect from_port="model" to_op="Apply Model (2)" to_port="model"/>
<connect from_port="test set" to_op="Apply Model (2)" to_port="unlabelled data"/>
<connect from_op="Apply Model (2)" from_port="labelled data" to_op="Performance" to_port="labelled data"/>
<connect from_op="Performance" from_port="performance" to_port="averagable 1"/>
<portSpacing port="source_model" spacing="0"/>
<portSpacing port="source_test set" spacing="0"/>
<portSpacing port="source_through 1" spacing="0"/>
<portSpacing port="sink_averagable 1" spacing="0"/>
<portSpacing port="sink_averagable 2" spacing="0"/>
<portSpacing port="sink_averagable 3" spacing="0"/>
</process>
</operator>
<connect from_op="Read CSV" from_port="output" to_op="Generate Attributes" to_port="example set input"/>
<connect from_op="Generate Attributes" from_port="example set output" to_op="Generate Attributes (2)" to_port="example set input"/>
<connect from_op="Generate Attributes (2)" from_port="example set output" to_op="Set Role" to_port="example set input"/>
<connect from_op="Set Role" from_port="example set output" to_op="Select Attributes" to_port="example set input"/>
<connect from_op="Select Attributes" from_port="example set output" to_op="Validation" to_port="training"/>
<connect from_op="Validation" from_port="model" to_port="result 1"/>
<connect from_op="Validation" from_port="training" to_port="result 2"/>
<connect from_op="Validation" from_port="averagable 1" to_port="result 3"/>
<connect from_op="Validation" from_port="averagable 2" to_port="result 4"/>
<portSpacing port="source_input 1" spacing="0"/>
<portSpacing port="sink_result 1" spacing="0"/>
<portSpacing port="sink_result 2" spacing="0"/>
<portSpacing port="sink_result 3" spacing="0"/>
<portSpacing port="sink_result 4" spacing="0"/>
<portSpacing port="sink_result 5" spacing="0"/>
</process>
</operator>
</process>
The results are better but the are still bad.
class prediction is 91.53 % (for high) and 33.47 % (for low). And low is the interesting part... :)
Any ideas?