Blog

Nutonian Piercing the Veil of Distortion Over Mission-Critical Images

Posted by Jay Schuren

8/12/14 10:00 AM

Imaging and advanced characterization are at the heart of a range of industries across aircraft component inspections, medical imaging, CPU manufacturing and more. As society continues to push the envelope in technological innovation, demand for the best quantitative characterization possible is always present. Distortion, or warping of image data, arises from the imaging equipment itself; common examples are fish eye lenses or fun house mirrors. A little-discussed issue is that changing the instrument settings often changes the distortion – limiting peak characterization performance across fields, reducing accuracy and escalating time and effort for discovery. 

Read More

Topics: Advanced Techniques, nutonian, Case study, Big data, U.S. Air Force

Predicting a Bond’s Next Trade Price with Eureqa: Part 2

Posted by Jess Lin

11/26/13 10:00 AM

In my previous post, we walked through the process of using Eureqa to predict the next price a bond will trade at. Starting with a massive spreadsheet with >760,000 rows and 61 columns, we were able to generate 11 equations to describe the data in 20 minutes. While I focused on just one of the equations, there is still more we can learn from Eureqa.

Read More

Topics: Advanced Techniques, Eureqa, Tutorial, Making predictions

Setting and using validation data with Eureqa

Posted by Michael Schmidt

6/28/13 4:02 PM

Eureqa automatically splits your data into groups: training and validation data sets. The training data is used to optimize models, whereas validation data is used to test how well models generalize to new data. Eureqa also uses the validation data to filter out the best models to display in the Eureqa user interface. This post describes how to use and control these data sets in Eureqa.

Default Splitting
By default, Eureqa will randomly shuffle your data and then split it into train and validation data sets based on the total size of your data. Eureqa will color these points differently in the user interface, and also provide statistics for each when displaying stats, for example:


All other error metrics shown in Eureqa, like the "Fit" column and "Error" shown in the Accuracy/Complexity plot, use the metric calculated with the validation data set.

Validation Data Settings

You can modify how Eureqa chooses the training and validation data sets in the Options | Advanced Genetic Program Settings menu, shown below:


Here you can change the portion of the data that is used for the training data, and the portion that goes into the validation data. The two sets are allowed to overlap, but can also be set to be mutually exclusive as shown above.

For very small data sets (under a few hundred points) it is usually best to use almost all of this data for both training and validation. Model selection can be done using the model complexity alone in these cases.

For very large data sets (over 1,000 rows) it is usually best to use a smaller fraction of data for training. It is recommended to choose a fraction such that the size of the training data is approximately 10,000 rows or less. Then, use all the remaining data for validation.

Finally, you can also tell Eureqa to randomly shuffle the data before splitting or not. One reason to disable the shuffling is if you want to choose specific rows at the end of the data set to use for validation.

Using Validation Data to Test Extrapolating Future Values

If you are using time-series data and are trying to predict future time-series values, you may want to create a validation data split that emphasizes the ability of the models to predict future values that were not used for optimizing the model directly.

To do this, you need to disable the random shuffling in the Options | Advanced Genetic Programming Options menu, and optionally make the training and validation data sets mutually exclusive (as shown in the options above). For example, you could set the first 75% of the data to be used for training, and the last 25% to be used for validation. After starting the search, you will see your data split like below:


Now, the list of best solutions will be filtered by their ability to predict only future values - the last rows in the data set which were not used to optimize the models directly. 

Read More

Topics: Preparing Data, Advanced Techniques, Eureqa, Tutorial

Working with discontinuous data in Eureqa

Posted by Michael Schmidt

6/28/13 4:02 PM

Several features in Eureqa assume that your data is one continuous series of points by default, such as the smoothing features and numerical derivative operators. This post shows how to tell Eureqa that there are breaks in the data.

Entering discontinuity with a blank row:

Read More

Topics: Preparing Data, Advanced Techniques, Eureqa, Tutorial

Using date and time variables

Posted by Michael Schmidt

6/28/13 4:01 PM

This post describes the best way to convert date or time values into numeric time values that can be used in Eureqa.

Time Values in Eureqa:
Eureqa can only store date and time values as numeric values (e.g. total seconds or total days). Therefore, you need to pick a reference point to measure a time duration from, and units to measure the time duration.

For example, you could convert a time value "8:31 am" to 8.52 total hours since midnight. Similarly for dates, you could convert a date like "Dec. 6, 1981 8:31 am" to 81.9 total years since 1900.

You need to make date and time conversions to numeric duration values in another program like Excel before entering into Eureqa (see below for example).

Pitfalls:

1) Do not concatenate date and time strings to get a numeric value. For example, do not convert a date like "1981-12-06" to 19811206. This representation of time is extremely nonlinear. It can preserve order, but has lost all meaning. Additionally, the values are very large and numerically unstable.

2) Avoid measuring time durations from a very distant reference point. For example, if you're data uses time values that span a few days, do not convert these time values to total seconds since the beginning of the century. The numeric values would be enormous and numerically unstable.

Instead, the best practice is to measure a time duration since the time point in your data set.

Convert in Excel:

Many programs can convert date and time values to numeric time duration values. In Excel, if you subtract two date cells, the result is the fractional number of days between the two dates. You could then convert days to hours or some other unit to get numeric values with reasonable numeric magnitudes. For example:
    =(A0-A$0)*24

and then repeated for all rows, would subtract the first date in cell A0, and multiply the resulting day values into hours.
Another useful function is the YEARFRAC function which converts the difference between a date and a reference date to the fraction of years difference between them. For example:
    =YEARFRAC(A$0, A0)
and repeated for all rows, returns the fractional value of years from cell A0.

See Also:
Read More

Topics: Preparing Data, Advanced Techniques, Eureqa, Techniques, Time Series

Custom error metrics and special Search Relations in Eureqa

Posted by Michael Schmidt

6/28/13 4:01 PM

Eureqa's Search Relation setting provides quite a bit of flexibility to search for different types of models. This post describes some advanced techniques of using the Search Relation setting to specify custom error metrics for the search to optimize; or more specifically, arbitrary custom loss functions for the fitness calculation.

Custom Fitness Using Minimize Difference

Eureqa has a built-in fitness metric named "Minimize difference". This fitness metric minimizes the signed difference between the left- and right-hand sides of the search relationship. For example, specifying:

y = f(x)

with the minimize difference fitness metric selected tells Eureqa to find an f(x) to minimize y - f(x). A trivial solution to this relationship would be f(x) = negative infinity. However, you can enter other relations that are more useful. Consider the follow search relation:

(y - f(x))^4 = 0

Here, the minimize difference fitness would minimize the 4th-power error. In Eureqa this setting looks like:


In fact, you can enter any such expression and the f(x) can appear multiple times. For example:

max( abs(y - f(x)), (y - f(x))^2 ) = 0

would minimize the maximum of the absolute error and squared error, at each data point in the data set.

Other Methods

There are many other possible ways to alter the fitness metric using the search relationship setting. For example, you could use a normal fitness metric (e.g. absolute or squared error) but scale both sides of the relation. For example, you could wrap each side of the search relation with a sigmoid function like tanh:

tanh(y) = tanh( f(x) )

Now, both the left and right sides get squashed down to a tanh function (an s-shaped curve that ranges -1 to 1) before being compared. This effectively caps large errors, reducing their impact on the fitness.

Even More Tricks

You can also use the search relationship to forbid certain values by exploiting NaN values (NaN = Not a Number). For example, consider the following search relation, which forbids models with negative values:

y = f(x) + 0*log( f(x) )

Notice the unusual 0*log(f(x)) term. Whenever f(x) is positive, the log is real-valued, and the multiplication with zero reduces the expression to y = f(x). However, whenever f(x) is negative, log(f(x)) is undefined, and produces a NaN value. Whenever a NaN appears in the fitness calculation, Eureqa automatically assigns the solution infinite error. Therefore, this search relationship tells Eureqa to find an f(x) that models y, but f(x) must be positive on each point in the data set.

This behavior can be used in other ways as well. Any operation that would produce an IEEE floating point NaN, undefined, or infinity will trigger Eureqa to assign infinite error. You can also add multiple terms like this to place multiple different constraints on solutions.
Read More

Topics: Advanced Techniques, Eureqa, Tutorial, Custom Error Metrics

Use time-delays or time-lags of a variable in Eureqa

Posted by Michael Schmidt

6/28/13 4:01 PM

A time delay retrieves the value of a variable or expression at a fixed offset in the past, according to the time ordering or index of each data point in the data set. This post describes the time-delay building-blocks available in Eureqa and different modeling techniques with delayed values.

Time Delay Building Blocks:

Eureqa provides the delay(x, c) building block to represent an arbitrary time-delay, where x could be any expression. The expression delay(x, c) returns the value of x at c time units in the past. When used as a building-block, Eureqa can automatically optimize expressions or variables to be delayed and the time-delay amount  c.

The figure above plots an arbitrary variable x and a delayed value delay(x, 1.0), where the values are ordered by some time variable t. The delayed version is equal to x at 1.0 time units into the past.

To use time-delay building-blocks, your data must have some notion of time or ordering. You also need to tell Eureqa which variable in your data represents the time or ordering value:


If you don't specify a time variable, Eureqa will use the row number in the spreadsheet as the time value of each data point.

If a particular delayed time value falls between two points in the data set, the value is linearly interpolated between the two data points using the time value.

Eureqa also provides the delay_var(x, c) building-block which is identical to delay(x, c), except that it only accepts a variable as input. It's provided as a special case of the delay(x, c) building-block to allow you to constrain the types delays used in the solutions. But in the end they are effectively identical.

Control the Fraction of Data Used for History

Notice that the delayed output plotted above does not have values on the left side of the graph for the first few time points. This is because these points request previous values of x that lie before the first point in our data set. Eureqa will automatically ignore these data points when calculating errors.

However, there is a way to control how much of the data set Eureqa is allowed to ignore - or effectively, specify a maximum delay offset. You can limit the fraction of data used for time-delay history values in the Advanced Solutions Options menu:


The default maximum fraction is 50% of the data. If you find that Eureqa is identifying solutions with very large time delays, perhaps just to avoid modeling difficult features in the first half of the data set, you may want reduce this fraction

Additionally, you can control the number of delayed values per variable (including a zero delay of an ordinary variable use) in this dialog.

Fixed Time-delays:

Another way to model a value as a function of its previous values is with fixed delays. You can enter in fixed time-delays, or "lags" of the variable, directly into the Search Relationship option. For example:

    x = f( delay(x, 2.1), delay(x, 5.6) )

This search relationship tells Eureqa to find an equation to model the value of x as a function of it's value at 2.1 and 5.6 time units in the past.

Minimum Time-delays:

You may also want to specify a minimum time-delay offset. If you entered a search relationship such as x = f(x), Eureqa would find a trivial answer f(x) = x. More likely, you wanted to find a model of x, but as a function of x at least some amount of time in the past. The way to do this is to again use a fixed delay, such as:

    x = f(delay(x, 3.21))

Here, 3.21 is the minimum time-delay. Now, if the time-delay building-blocks are enabled, Eureqa can delay this delayed input further if necessary.

Delay Differential Equations:

Another common use for time-delays in for modeling using Delay Differential Equations. Finding delay differential equations is just like searching for ordinary differential equations. For example, entering a search relationship like:

    D(y,t) = f(y)

but also enabling time-delay building blocks. This relationship has a trivial solution however: Eureqa will return the slope formula such as

    f(y) = ( y - delay(y, 0.1) )/0.1

Therefore, you most-likely want to limit the total number of delays per variable to one (which includes the zero delay of the normal variable use). You can set this in the Advanced Solution Settings menu. The default is unlimited.

Implementing Delays Outside of Eureqa

In Matlab, you can implement a time delay using the interp1 function. For example, the expression delay(x, 1.23) would be implemented as:


    interp1(t, x, t - 1.23, 'linear')


Implementing delays in Excel is a littler harder. You need to download an Excel add-on that adds an interpolate function. For example, the package XlXtrFun adds a function "Interpolate" that is just like Matlab's interp1. There are also other guides for Linear Interpolation with Excel.
Read More

Topics: Advanced Techniques, Eureqa, Tutorial, Techniques, Time Series

Modeling a derivative such as velocity or acceleration

Posted by Matt Fleming

6/28/13 4:00 PM

Eureqa can automatically estimate numerical derivatives in order to model the rates of change of variables in your data. Often derivatives are more natural and simpler for modeling certain types of phenomena, particularly in physics. This post discusses the basics of entering derivatives into the Eureqa search relationship.

The Derivative Operator:

Eureqa provides the derivative operator D(x, y, n) where x and y are any arbitrary expressions and n is an integer representing the order of the derivative to take. This operator can be used in the Search Relationship setting. For example, consider the search relationship:

    D(x,t,1) = f(x,t)

This relation tells Eureqa to find a function of x and t that models the first derivative (e.g. a velocity or slope) of x with respect to t. Short-hand for the first derivative is D(x,t). The derivative operator can also appear inside the formula as an input variable, for example:

    D(x,t,2) = f( x, D(x,t) )

This relation tells Eureqa to find a model of the second derivative (e.g. an acceleration or curvature) of x with respect to t, as a function of x and the first derivative of x. In Eureqa, this relation will appear as:


Eureqa displays the derivatives in mathematical format after the relationship text is entered.

Alternatively, you could estimate the numerical derivatives ahead of time using another program, and enter these values as a new variable in the data set rather than using Eureqa's derivative operator.
Starting the Search:

Eureqa will calculate the numerical derivatives that appear in your search relation when you start the search. The following screen will appear after you click start:


Eureqa estimates the numerical derivative using a spline fit to the data. This allows more accurate derivative estimates than other methods in case the data contains noise.

Advanced:

Estimating numerical derivatives accurately is a challenging task when the data is sparse or contains noise. Eureqa's derivative estimation is an improvement over the most basic methods like Newton's difference quotient. However, it does not work well in all cases.

One particular problem with spline curves is their accuracy at the head and tail of the data - these points are "surrounded" by fewer data points and thus have higher estimation error. If you can, you might want to ignore these points entirely using a weight variable. Simply add a new column to your data, and set the weight to 1 for all data points but near zero for the first and last 5 to 10 data points.

It may also be worth the effort to estimate the numerical derivatives outside of Eureqa using more specialized tools. For example, you may want to compute the derivative values in R or using Matlab's spline toolbox, and then paste these into Eureqa as a new column variable.
Read More

Topics: Advanced Techniques, Eureqa, Tutorial, Techniques, Numerical Derivatives

Modeling a binary yes/no output

Posted by Michael Schmidt

6/28/13 3:58 PM

Binary classification attempts to predict a variable that has only two possible outcomes - for example, true or false, or buy or don't buy. This post describes how Eureqa can be used to model a boolean decision or classification value.

Binary classification is also one of the most widely studied problems in machine learning, and there are many optimized approaches for prediction (e.g. neureal nets, support vector machine, etc). Using Eureqa for classification (or symbolic regression in general) has a few advantages:
  • finding models requires less data
  • models can extrapolate extremely well
  • resulting models are simple to analyze, refit, and reuse
  • the structure of the models gives insight into the classification problem
The last point is the most important in my opinion - not only can you predict but you can also learn something about how the classification works, as in the example below. This isn't possible with most other methods, but comes at a cost of increased time to find an analytical solution if one exists. Here's how to do it in Eureqa.

Squash Method:

The key to this method is to tell Eureqa to search for equations that tend to be negative when the output is false, and positive when true. We then put solutions inside a step function to obtain outputs of either 1 (true) or 0 (false).

Step 1: Eureqa works with numerical values, so define true outcomes to have value 1, and false outcomes to have value 0. Now, enter in the boolean variable into Eureqa as a column of 0 and 1 values.

Step 2: We want to find formula that predicts 0 and 1 values. One way to do this is to tell Eureqa to search for an equation that goes inside a step function before comparing with the boolean value. For example, we could enter "z = step(f(x,y))" into the search relationship setting, where z is a boolean value we want to model, x and y are other variables in the data set, and f(x,y) is the formula that Eureqa attempts to find. The step function is a built-in function in Eureqa that outputs 1 if the input is positive, and 0 otherwise. In other words, we are telling Eureqa to find equations that tend to be negative when z is 0 (false), and positive when z is 1 (true).

Step 3: Start a Eureqa search as normal. Eureqa reports equations for f(x,y) which is inside a step function. To use these solutions to predict the boolean value outside of Eureqa, we need to substitute the formula back into the search relationship. In other words, remember to place the reported solutions back into a step function to obtain the final model.

Example:

Let's say we collected the following data, where x and y are two input variables, and z is a boolean outcome that we want to model (red = true, green = false):

We enter in a search relationship as "z = step( f(x,y) )":


We then start the Eureqa search. After a few minutes, Eureqa identified a very accurate solution:

    f(x,y) = 1.98 + 2.02*x*y - 3.05*y*y - x*x

You may recognize this equation as a tilted ellipse. Plotting this solution on the data makes this clear:
Here, we used Eureqa to identify a boolean model of whether a data point would be red or green based on the 2D location of x and y. The resulting solution shows that the data can be separated by an ellipse.

Advanced:

Another type of squashing function is the logistic function which varies smoothly between 0 and 1. It provides a better search gradient than the step function which has almost none. For example, we could enter a search relationship instead as:

    z = logistic( f(x,y) )

A side effect is that logistic(f(x,y)) can produce intermediate values, such as 0.77 or 0.001. Therefore, we would need to threshold this value to get final 0 or 1 outputs. A simple way to threshold at 0.5 is to simply replace the logistic with a step function for the final step to make final predictions of the boolean value.
Read More

Topics: Advanced Techniques, Classification, Binary Output

Modeling outputs that have a range of values

Posted by Michael Schmidt

6/28/13 3:57 PM

Often you might want to specify that the output of a model should fall within a certain range rather than an exact numerical value. This post shows one way to do this with Eureqa. The goal it so find the simplest equations who's outputs always lie between some min/max value for each data point.

Enter Min and Max Values for each Data Point:

Step 1: For each data points that you only have a range of output values (the min and max values), you simply need to add two rows for that data point, one with minimum value and one with the maximum value (keeping all other variables in the row the same).

Step 2: Next, set the fitness metric to the "Mean Absolute Error" option.

Step 3: Start the Eureqa search as usual. Solutions that fall between the min and max values will have identical absolute error.

If a model output lies between the min and max values, the absolute error happens to be indifferent (mathematically) to where exactly this value lies. If the value moves closer to the max value, the error on the max value data point decreases linearly, but the error on the min value data point increases linearly also.

Example

In Eureqa, your data view should look similar to:


 Where each input x is repeated twice, once with the minimum y value and again with maximum y value.

We can then start the search using the Mean Absolute Error fitness metric, and get various solutions that fall into the min/max ranges:


These solutions may have slightly different fitness values because some min/max data point pairs might get separated between the train and validation data sets. One way to avoid this is to change the train and validation sets to use all data or not shuffle their points in the Advanced Genetic Program Settings menu.

Using Separate Min and Max Values in a Custom Error Metric

Another option is to specify a custom error metric in the Search Relationship, this allows you to enter your min and max range values in different columns. For example, consider the following search relationship:

abs(y_min - f(x)) + abs(y_max - f(x)) = 0

where x is the input, and y_min and y_max are two different variables representing the min and max values of the range of outputs for each input x. The custom error in this relation is equivalent to the previous method.
Read More

Topics: Advanced Techniques, Eureqa, Modeling Outputs

Subscribe to Our Blog!

Follow Me

Posts by Topic

see all