Shift error

Jose asked 11 months ago

Hi from Spain.

I’m teaching myself machine learning and deep learning in python, and I am now much further into it than I expected months ago, thanks in part to your online posts. So thank you very very much.

Sometimes I come across funny results that sometimes I disregard or I finally understand. This time I found a model behaviour that is haunting me.

The subject is about conv2d over a dataset I prepare using consecutive rows and a lookback l=20; this is, if xij is a row (x=value, i=feature, j=row). My “images” are like this:

x11, x21, x31,…
… (l rows)
x1(1+l), x2(1+l),x3(1+l)

for target y(1+l+1) adding 1 so I can predict the next row’s y

next frame

x1j, x2j, x3j …

x1(1+l+j), x2(1+l+j),x3(1+l+j)

for target y(1+l+j+1)

(there’s probably a notation error, but you get it)

The problem is that it doesn’t work when it should, It works when it shouldn’t: in i+l .

I mean, during the testing, the accuracy is very good, but not for the correct moment in time, but l leaps in the future:

if Y is actual event and y is prediction, the ideal situation would be Y(1+l+j+1) = y(1+l+j+1) = predict(x1(1+j),… | x1(1+j+l) )

but it is accurate l leaps later, when it is not needed anymore,  Y(1+l+j+1+l) = y(1+l+j+1) = predict(x1(1+j),… | x1(1+j+l) )

The damn thing nails it if I take the predictions with an offset 20 leaps in the future; If it was 20 leaps in the past, fine, I’d take a result 20 leaps back in the log, … but in the future? And nailing it? Bam! I know it is not by chance, and I’m messing up something.

Let's pause for a second! 👩‍💻

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This is how I build the frames:

look_back = 20

if normalize:
scalerx = MinMaxScaler(feature_range =(-1, 1))
scalerx =
x_train = scalerx.transform(x1)
x_test = scalerx.transform(x2)
y_train = y1
y_test = y2
x_train = x1
y_train = y1
x_test = x2
y_test = y2

def reshapexy(xp,yp, look_back, shift):
 global normalize
 if normalize:
  x = xp
  x= xp.to_numpy()

 y= yp.to_numpy()
 dataX, dataY = [], []
 offst = look_back+shift
 rng = len(x)-offst
 for i in range(rng):
  a = x[i:(i+offst)]
 return np.array(dataX), np.array(dataY)

x_train_r, y_train_r = reshapexy(x_train,y_train,look_back,shift)
x_test_r, y_test_r = reshapexy(x_test,y_test,look_back,shift)
y_train_rh = categorize2(y_train_r,True)
y_test_rh = categorize2(y_test_r,True)
x_train_rh = tf.expand_dims(x_train_r, axis=-1)
x_test_rh = tf.expand_dims(x_test_r, axis=-1)

k4model.add(Conv2D(num_features, (num_features,5), activation=’relu’, data_format=’channels_last’,padding=’same’, input_shape=(look_back,num_features,1)))
k4model.add(Dense(2, activation=’softmax’))
k4history=,y_train_rh, epochs=epochs, batch_size=batch, shuffle=False, class_weight = cws_dict_full,verbose=0)
y_k4pred = np.argmax(k4model.predict(x_test_rh), axis=-1)

Chris Staff replied 11 months ago

Can it be the case that this isn’t the full code?

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