Mass-Spring Oscillations Lab

In this lab, we were going to find factors effecting period of the spring oscillation. Professor wolf asked us to be two team, and each team had four groups to measure spring constants of different springs and their mass.
First of all, we need to measure all spring constant of them. We determined the original length of spring S1, and put a 0.05 kg mass on the spring and measure the length of spring S2. Thus, the spring constant is (0.05kg*9.8m/s^2)/(S2-S1).
spring constant and mass:
1. 2.44 N/m 9.7 g
2. 6.5 N/m 11.1 g
3. 23.3 N/m 27.7 g
4. 18.52 N/m 19.9 g
In my group,our spring is the first one in the list.We hanged 106 g mass on the spring and make it rock. And then, we use computer to collect its motivation.

From the graph, we can find the period 1.6 s.
After four groups finishing the experiment, we showed the data to each other.
spring  spring constant     mass     mass add to the 100 g hanging mass  period
     1         2.44 N/m    9.7 g                                    6 g   1.6 s
     2          6.5 N/m   11.1 g                                    5 g 0.814 s
     3         23.3 N/m   27.7 g                                    0 g  0.42 s
     4        18.52 N/m   19.9 g                                    3 g  0.47 s
From this table, we made a graph to directly show the relationship.

Conclusion,as the mass being bigger, the period will be smaller.

As the spring constant being bigger, the period will be smaller as well.
And what we predict,the period will be smaller when the spring constant being bigger, but the period will be bigger when the mas increases, so I confused about this.
If the value of spring constant were 5 % off, the period will be 2.34% increases.

A high value of spring constant spring means a greater force in Newton's second law, F = ma. For a given mass, that means a greater acceleration so the mass will move faster and, therefore, complete its motion quicker or in a shorter period.

Newton's second law, F = ma or a = F/m , tells us that a larger mass will have a smaller acceleration (for the same force) so that a greater mass will simply move slower and, therefore, take a longer time to complete its motion.